You might have a load of crap trapped in there blocking the vertical roller. Take the ball out and see if you can manually roll the vertical roller. If not, you probably need to give it a good clean.
In any event, it's always worthwhile opening them up from time to time and giving them a good clean inside. You might even have got some crap trapped in the little wheels that the rollers turn. These wheels have slots in them at regular intervals that let IR light from a LED pass through to a photodetector. The resulting train of pulses are what the internal circuitry use to measure movement.
I used to get a similar problem caused by direct sunlight hitting my mouse. It turns out the casing allowed enough light through to trigger the above photodetectors which for some unknown reason were not shielded from ambient light.
