i installed a driver update a bit ago and the pressure is not working in paint tool sai
it was working before the update
Ahh yes, that glitch :P Since I've never owned a cintiq myself, don't hold this as a surefire solution but...
I had a similar situation with my intuos P&T and the solution was uninstalling it completely and then downloading it again.
For some reason several programs (like gimp in my case) won't "adjust" themselves and see the tablets as a mouse for no reason.