Scenario: I am a first time DM, playing with two of my best friends, who are first time players. One of them plays a Dwarf Cleric, the other one a Wood Elf Fighter. We played "Dragons of Stormwreck Isle" and have transitioned into "Curse of Strahd". The cleric player enjoys the combat since they can be useful in many ways (ranged/magic damage, healing, melee attacks), however the fighter player does not.
I gave them the option to upgrade to a +1 Longsword, which they did, but they still miss a lot of attacks, which understandingly frustrates them. Often, when they do hit, they deal little damage, which frustrates even more. What sucks even more than that, is missing 2 attacks, then using action surge and missing the next 2 attacks as well, using Inspiration reroll and missing once again (this happened last session; 5 missed attack-rolls against an AC 15 enemy in one round).
Before the last combat encounter I gave them some tools, which help specifically against the enemies they fought, but they forgot to use them. They even asked an NPC for tools against vampires, the NPC gave them a wooden stake and they fought within 2 minutes of getting it, but still forgot to use it.
My enemies were surprised by the attack, but developed the goal of escape, preferably while kidnapping a NPC, which the players were controlling in combat. I try to give all of the enemies some sort of motivation, and try to make them use more than just their most powerful attack every round.
How do I encourage players to use items and strategy in combat, instead of just standing there and saying "I attack"?