Modifier Keys stuck, glitchy (photoshop cc 2018)

Hi all,

I have been having this problem with Astropad forever. Whenever I press the modifier keys ( for example to proportionally size a circle with shift, or command select layers) it doesn’t stay selected, and flashes on and off. So I can’t use it, and have to use my computer to use modifier keys which is wasting time and hurting my back.
I dont have wacom driver installed, and I have an updated astropad, mac and iPad pro.
I dont know how to fix this, please help

thanks