I have my curve for three months now, but every once in awhile the trackball will get stuck and won't move. I try cleaning it. Sometimes it helps and sometimes it doesn't. Then after minutes later it goes back to working like it didn't stop working for a minute.
Try following what ballinsol recommended. You could have lint or God knows what stuck under there. Another great trick is to clean the ball itself with a Q tip and a small amount of rubbing alcohol. Also try a battery pull. Whenever I had trackball issues with my old Pearl, this did the trick.
I had trackball issues with my curve when I had one also. I just removed the battery. I think battery pulls fix everything lol. It worked though I also tried the rubbing alchol