It's possible that there's some dust or other gunk on the rollers inside the mouse that the mouse ball moves against. Remove the ball, clean it, clean the rollers (a small pencil eraser works well, and doesn't damage the surface) and try that.
I get this problem with my laptop about every 6 months or so.
I use a vacuum cleaner with the smallest nozzle on the keyboard to suck out any dust or crumbs, that fixes it. You could also open it up if you have a tiny screwdriver to really get it clean.