OS X :: Why Mouse Scroll Ball So Intermittent

I love the mouse (is it the mighty mouse?) which comes with the imac. The little scroll ball is brilliant especially for say scrolling down in itunes. But I always find that sometimes it won't work in a variety of different ways. i.e.

1) Sometimes it will let me scroll down but not up
2) sometimes up but not down
3) sometimes not up or down
4) occasionally it works both up and down.

As I type this I can go down but not down. I've had this problem on both our older IMac and now our new one. It can't be dust and muck and the ball does rotate. Why can't the ball just work all the time.