my mouse quits scrolling one way

Check the quote from Apple support document below.

"If the scroll ball on your Mighty Mouse has become discolored or dirty, use a clean, lint-free cloth lightly moistened with water to clean it. Wipe the ball and the surrounding area, making sure to rotate the ball itself to ensure complete coverage. If the scrolling feels rough or if the scroll ball isn't scrolling up, down, or side-to-side, hold the Mighty Mouse upside down and roll the ball vigorously while cleaning it to help dislodge any particles that might have collected on the internal hardware."