If it's WACOM then you have to recalibrate your tablet or screen.
I'm using the Cintique 24" Pro with two other monitors and I have to recalibrate every once and a while.
1. Open Wacom Desktop Center
2. Recalibrate pen to monitor
3. Select recalibration on new window that pops up
4. click pen tip to 4 crosshairs that pop up in seqence to 4 corners of monitor