If you're talking about the little scroll ball then it could be that dust got stuck inside it. I've heard different solutions to this. Basically it just needs to be cleaned and I think that rubbing alcohol is the best solution. You can also use compressed air. Be sure not to over saturate the cloth when using alcohol. Just get it slightly damp and roll it on the ball. Make sure you move the ball all the way around to get everything. If you use compressed air you just move the ball while blowing the air into it.