Mac Pro not recognising cursor click on second monitor

I'm using two Apple 22 inch displays (approximately 8 years old), connected via thunderbolt to a late 2013 Mac Pro 3 GHz 8-Core Intel Xeon E5.

I've noticed the cursor clicks are not registering on the second screen in certain software (specifically Avid Media Composer & Adobe After Effects). Basically, I can't point and click on any radio buttons on my second screen, even though the cursor is correctly positioned on them (and I can click & drag on the menu bar of any window to make it active & move it).

I've noticed this problem on another pair of apple 27 inch cinema displays that I also use with this CPU.