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Mine does that sometimes too. It seems to stick or something.

If I push down on the scroll ball and roll it up and down quickly for a few seconds it seems to unstick it and then it works fine again.

Chris
 
Don't mean to be patronizing, but have you tried cleaning it, ie rolling the track ball over paper/cloth?

Follow Slip's advice. Flip it over and, pressing hard, roll the ball in all directions over a piece of paper. 30 seconds of this or so should fix your problem.
 
The problem is usually a result of dust or dirt getting caught between the sensor and the ball, hence why turning it upside down would work; it would knock the dust out of the way of the sensor (usually).
 
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