I have had this very same issue before.
you may need to turn your trackballs sensitivity up
(options>screen/keyboard> then scroll down to trackball)
but...
you may have a dirty trackball. had you had your berry for a while?
if you have and the ball is no longer a white color check out this thread
http://forums.pinstack.com/f184/trac...05/#post890754
hope it helps!
If you follow the link and do decide to clean your trackball I want to reinforce that if you do this, and your phone is still under warranty, this will void your warranty. just FYI. my phone is old and has no warranty so I'm not worried about it.