The first fix recommended is to firmly press and roll the trackball in the direction that it does not want to move. This is will often clear any dust, lint and debris that is hindering it.
If you have cleaned it already and are still experiencing this, you may be in for a new trackball. They are readily available at
HorizonWirelessOnline.com - Horizon Wireless, The World’s Cellular Repair Center ß, and easy to replace.