You might first check to see if it needs cleaning.
Take the ball out and check the small rollers that the ball rests against.
If these have crud (dirt) on them, use a small knife and scrape them clean
and then alcohol on a "Q-Tip" to clean them.
A small amount of dirt on these rollers can cause a lot of problems.
If you have the chance, borrow a mouse and try it.  This can help you
determine if the mouse or the computer is at fault.