Press down firmly and roll it around in the direction(s) that it is sticking in or try spraying canned air into it to clean it.
You could try cleaning it thoroughly:
http://www.blackberryforums.com/gene...pic-guide.html
OR
Buy a new one from Horizon Wireless (Forum sponsor, find them in the sponsor section) and replace it. They are relatively easy to replace, here is a guide:
YouTube - HOW TO FIX REPAIR BLACKBERRY TRACKBALL JOYSTICK FOR PEARL 8100 8110 8120 8130