Greasy roller.
Sticky bearing(s).
Encoder wheel warped or loose on end of roller.
Dirty optics.
Faulty optics
....the list goes on..
.....
Crack that sucker open and give it an inspection.
If nothing obvious, swap the right/left optics with the up/down and see if the problem has moved with it.
If still left/right problem, and the encoder wheel is moving fine, check wire connections, continuity , etc,
Degrease everything and lube bearings as required while you're in there.
Have fun.
How is it connected to your PC? (built-in usb&ps/2 interface, Opti-Wiz, Opti-pac?)