Assuming you're within your "grace period" I'd take it back and have it exchanged for another one. Have the techs check it out (explain to them exactly the problem that you're having) and they more than likely will be more than likely to take care of you on the spot.
A can of "canned air" works well. Blow off the ball and move it around. Sometimes the ball will get gooey and dusty and not want to work. I've also used the edge or corner of a paper card to work in and around the ball to get all the gunk out. Pulling the battery and resetting the unit often times helps cure the problem if your sure the trackball is good and clean. What OS are you running? My tmobile OS that came on the Pearl seemed to have this problem more than the upgraded versions. You can check under options/about.
