Depending on the arrangement of the two monitors in you'reyour monitor settings - how exactly they align with each other - you might have to move you'reyour pointer up or down in order to get to the other monitor. Especially if the two monitors don't have the same resolution, butalthough it can be the case with the same resolution as well.
Here is an example of a two monitor-monitor setting under Windows 10. If you are on the lower right corner of screen 1 (anywhere in the red area) you wontwon't be able to move to the second screen. You will have to move the mouse pointer further up - out of the red area - in order to move to the 2nd monitor: