we find that most cases you can clean it from the outside by rolling the ball with a cleaning substance. But while doing this cleaning method does pull out some dirt that stops the ball mechanisms from rolling (there are 4 directional pins inside), the pins eventually get too clogged up to roll.
The only way of fixing it after that is to open it and clean it from the inside. it can be done, but will require cracking the lower frame off the mouse, then re-glueing it afterwards