Apple’s Mighty Mouse works great most of the time, but there are times when the trackball simply stops working. Sometimes you can scroll up but not down, down but not up, and sometimes, you can’t even scroll at all.
There are a number of ways to fix this, but not all of them work swimmingly. For instance, you can try rubbing the trackball with a q-tip or paper towel soaked in rubbing alcohol, or you can try inserting the corner of a piece of paper into the space between the trackball and the rest of the device. From our experience, both of the above solutions work, but only temporarily.
That being the case, sometimes you need to get your hands dirty and actually open up your mouse and clean the trackball manually to get rid of any dirt and dust particles that might have taken up residence there. The following video explains how to do just that. Lastly, and in typical Apple fashion, opening up the mouse requires some effort, so proceed with caution and at your own risk!
Related: Does Apple’s Mighty Mouse suck?