The first thing to do is check Pen Pressure is switched on, Brush Palette > Shape Dynamics > Jitter, and select Pen Pressure from the drop down menu. This one:

The next thing to try is reinstalling the driver.
The first thing to do is check Pen Pressure is switched on, Brush Palette > Shape Dynamics > Jitter, and select Pen Pressure from the drop down menu. This one:
The next thing to try is reinstalling the driver.