It happens when dust and dirt get in there. Just like an old fashioned mouse.
Try a can of compressed air to blow out the gunk. You can also turn off the device and push down hard while moving the ball around. That will sometimes loosed up the dirt.
You can take the trackball right out for cleaning there are 3 clips behing the round case that the track ball sits in. With a small driver this can be poped off allowing a good cleaning
If you search the internet there is a step by step guide with pictures somewhere on removing the ball to clean it. That is one thing I am not looking forward to when I finally upgrade by BB.
Yeah I was having the same problem (trackball wouldn't go down at all) but I did a search and found pictures on how to take the assembly apart and clean it. It took maybe 10 minutes and it was working like brand new!
My trackball is only scrolling side to side and I can't go up or down. This is frustrating.
It happened abt a month ago and suddenly it starting working properly and today its giving problems again.
Posted from my CrackBerry at wapforums.crackberry.com
There are a number of people who swear that moving the trackball around while holding your device facing trackball-side-down works wonders. I think I'd try that before grinding whatever grit is in there deeper into the track.
Trackball Sticking 
Linear Mode
