There is no ball in the scroll wheel portion of most mice these days. The one exception is the Mighty Mouse from Apple.
It is usually some sort of spring-loaded mechanism and they wear out. I've had to replace several mice over the years when the scroll wheel mechanism bit the dust.
If you want to try to repair it yourself, here are some videos that show how to repair the scroll wheel on two types of mice.
You can find other videos for more types of mice on youtube.