My wife's Mighty Mouse recently stopped scrolling down. Scrolling up was fine, scrolling sideways, no problem, but scrolling down...nothing.

It seems that a number of others have experienced the same problem with the scroll ball as evidenced here and here.

Fiddling around with the ball, I could feel some minor feedback in all directions but down. Suspecting dirt or oil preventing the ball from engaging whatever it is that it engages, I moistened a Q-Tip with isopropyl alcohol and swabbed the ball in a circular direction for about 15 seconds. After this, no more problems.