There are several reasons.
First off, a palantir does not allow the user to see the future. They only allow you the present and past. (There's another error in your question, which is that Sauron controlled the Ithil-stone, not the Witch-King; that's addressed below.)
A palantir is video only, no audio, unless being used to communicate with another palantir, and even then it's really the transmission of formalized thought, not audio. So Sauron would not have been able to hear their plan. He would not have known what they intended at the Council of Rivendell, or at any time afterward.
Alone the palantiri could only ‘see’: they did not transmit sound.
By themselves the Stones could only see: scenes or figures in distant places, or in the past. These were without explanation; and at any rate for men of later days it was difficult to direct what visions should be revealed by the will or desire of a surveyor. But when another mind occupied a Stone in accord, thought could be ‘transferred’ (received as ‘speech’), and visions of the things in the mind of the surveyor of one Stone could be seen by the other surveyor.
—Unfinished Tales Part Four, "The Palantiri"
You have to know where (and probably when) to look, and the world is a big place. In fact, you have to actually orient yourself around the Stone to look 'through' it in the direction of what you want to see, then you have to focus on the right thing, when is a wearying task, even for the rightful owners.
A surveyor, therefore, who wished to look west would place himself on the east side of the Stone, and if he wished to shift his vision northward must move to his left, southward. [...]
Ungoverned by a directing mind they were wayward, and their ‘visions' were (apparently at least) haphazard. From a high place their westward face, for instance, would look to vast distance, its vision blurred and distorted to either side and above and below, and its foreground obscured by things behind receding in ever-diminishing clarity. [...]
A viewer could by his will cause the vision of the Stone to concentrate on some point, on or near its direct line. The uncontrolled ‘visions' were small, especially in the minor Stones, though they were much larger to the eye of a beholder. [...]
But controlled by the will of a skilled and strong surveyor, remoter things could be enlarged, brought as it were nearer and clearer, while their background was almost suppressed. Thus a man at a considerable distance might be seen as a tiny figure, half an inch high, difficult to pick out against a landscape or a concourse of other men; but concentration could enlarge and clarify the vision till he was seen in clear if reduced detail like a picture apparently a foot or more in height, and recognized if he was known to the surveyor. Great concentration might even enlarge some detail that interested the surveyor, so that it could be seen (for instance) if he had a ring on his hand.
But this ‘concentration’ was very tiring and might become exhausting. Consequently it was only undertaken when information was urgently desired, and chance (aided by other information maybe) enabled the surveyor to pick out items (significant for him and his immediate concern) from the welter of the Stone's visions.
—Unfinished Tales Part Four, "The Palantiri"
Frodo did not openly wear the Ring, so Sauron likely would not have been able to see it on his person, excepting the times that he brought it out. The palantir cannot see in the dark.
Also, what they ‘saw’ was directed or hindered by chance, by darkness, or by ‘shrouding’ (see below). The vision of the palantiri was not ‘blinded’ or ‘occluded’ by physical obstacles, but only by darkness; so they could look through a mountain as they could look through a patch of dark or shadow, but see nothing within that did not receive some light. They could see through walls but see nothing within rooms, caves, or vaults unless some light fell on it; and they could not themselves provide or project light.
—Unfinished Tales Part Four, "The Palantiri"
Frodo, as we know, kept the Ring inside his tunic, so it would not have been visible.
He stooped. Very gently he undid the clasp at the neck and slipped his hand inside Frodo's tunic; then with his other hand raising the head, he kissed the cold forehead, and softly drew the chain over it.
—The Two Towers, "The Choices of Master Samwise"
Sauron could not and did not look into the palantir all the time.
It must also be considered that the Stones were only a small item in Sauron's vast designs and operations: a means of dominating and deluding two of his opponents, but he would not (and could not) have the Ithil-stone under perpetual observation. It was not his way to commit such instruments to the use of subordinates; nor had he any servant whose mental powers were superior to Saruman's or even Denethor's.
—Unfinished Tales Part Four, "The Palantiri"
There's also a suggestion that the palantiri were range-limited:
The greater palantiri could look much further than the lesser; for the lesser the ‘proper distance’ was of the order of five hundred miles, as between the Orthanc-stone and that of Anor.
—Unfinished Tales Part Four, "The Palantiri"
Since the Ithil-stone was one of the lesser stones, Sauron would not have been able to track the Fellowship for most of their journey.
There is one time Sauron very nearly did catch on: when Frodo was on the Hill of Seeing. But Sauron did not find Frodo, thanks to Gandalf's intervention. The text is not explicit, but I don't think we can understand here anything other than that Frodo's use of the Ring in a 'magically' significant place attracted the notice of Sauron using the palantir, who tried to use it to track the Ring-wielder down. (Perhaps Sauron was not aware the Ring was actually in use, but just knew that something was going on here.)
At first he could see little. He seemed to be in a world of mist in which there were only shadows: the Ring was upon him. Then here and there the mist gave way and he saw many visions: small and clear as if they were under his eyes upon a table, and yet remote. There was no sound, only bright living images. The world seemed to have shrunk and fallen silent. He was sitting upon the Seat of Seeing, on Amon Hen, the Hill of the Eye of the Men of Númenor. Eastward he looked [...]
And suddenly he felt the Eye. There was an eye in the Dark Tower that did not sleep. He knew that it had become aware of his gaze. A fierce eager will was there. It leaped towards him; almost like a finger he felt it, searching for him. Very soon it would nail him down, know just exactly where he was. Amon Lhaw it touched. It glanced upon Tol Brandir - he threw himself from the seat, crouching, covering his head with his grey hood.
He heard himself crying out: Never, never! Or was it: Verily I come, I come to you? He could not tell. Then as a flash from some other point of power there came to his mind another thought: Take it off! Take it off! Fool, take it off! Take off the Ring!
The two powers strove in him. For a moment, perfectly balanced between their piercing points, he writhed, tormented. Suddenly he was aware of himself again. Frodo, neither the Voice nor the Eye: free to choose and with one remaining instant in which to do so. He took the Ring off his finger. He was kneeling in clear sunlight before the high seat. A black shadow seemed to pass like an arm above him; it missed Amon Hen and groped out west, and faded. Then all the sky was clean and blue and birds sang in every tree.
—The Fellowship of the Ring, "The Breaking of the Fellowship"
‘Yes,’ said Gandalf, ‘that was Gwaihir the Windlord, who rescued me from Orthanc. I sent him before me to watch the River and gather tidings. His sight is keen, but he cannot see all that passes under hill and tree. Some things he has seen, and others I have seen myself. The Ring now has passed beyond my help, or the help of any of the Company that set out from Rivendell. Very nearly it was revealed to the Enemy, but it escaped. I had some part in that: for I sat in a high place, and I strove with the Dark Tower; and the Shadow passed. Then I was weary, very weary; and I walked long in dark thought.’
—The Two Towers, "The White Rider"
So, in summary: Sauron did not discover their plan with the palantir because there was nothing discoverable by the palantir. The plan to destroy the Ring was spoken of at the Council, and at a few other occasions, but Sauron could not have discovered the plan from speech. Unless the plan had been written down on a big sign, Sauron couldn't have figured anything out.
He possibly could not have spied on the Council or the Fellowship anyway, until they came "in range" of his palantir. And at that point, he would have had to find the Fellowship among all the other hundreds of thousands to millions of people in the region, and still would not have been able to see who of them possessed the Ring, given it was concealed on Frodo's person. And if he had been able to find them, he would have assumed it was Aragorn, not Frodo - the heir of Isildur, not a lowly Hobbit - why would the Hobbit still be left with an instrument of 'command and domination'?
At that point, Sauron would have been intently focused on Aragorn - which he was, after Aragorn's challenge in the palantir and his attack at the Field of Cormallen. Sauron would have used every minute of palantir-time to figure out what Aragorn was doing and thinking since every piece of information available to him would have suggested that Aragorn most likely had the Ring.
