Wacom pen pressure sensitivity always turns off in Photoshop

I got this issue since I got my new windows system. 

I do my daily Photoshop session with my Wacom Intuos 4, then randomly later, sensitivity of the pen turns off. The only way to recover the problem is to reboot Photoshop and sometime the whole system. 

Any idea why it doing that?

I downloaded the latest Wacom drivers and Photoshop CS4 is up to date and the problem still persist.