I love my Mighty Mouse. It feels great in my hand, the scroll ball is absolutely a joy to use, and it tracks extremely well in OS X. The downside is that the scroll ball has a tendency to get dirty over time, and it can impair its ability to scroll. My Might Mouse started to suffer these problems, and after a couple of days, lost the ability to scroll down altogether.
A quick search of the web yielded a few different options on how to clean it. One of them takes it to the extreme and shows you how to dismantle the mouse and clean the internal parts individually. Thankfully, that wasn’t necessary for my mouse.
In short, I tried rolling the ball while wiping it with a slightly damp cloth. It didn’t help much at all. On Apple’s suggestion, I turned the mouse upside and performed the same action. Bingo. That did the trick.
So, the next time your Mighty Mouse stops scrolling, turn it upside down and give the scroll ball a few pushes. And if that doesn’t work, you can always disassemble it.