Split screen problems

Hello, all

As an online teacher I very often use split screen in my lessons but I have a very strange difficulty with it, I hope people can advise me.

Sometimes, I ask the first program to display on the left (or right) and it works, but when I want to select the other program to display alongside it, it very often tells me that the other program is 'not available in this view'. Then, I ask it again, sometimes via the other program, and it invariably just lets me do what I originally wanted to do, without ANY changes in what programs are open. Typically I need to ask 2 or 3 times, maximum 4 times, but it has never outright refused to let me split the screen, I just need to ask it a few times.

It is solvable but annoying, can anybody suggest why this might be happening?

Thanks!