Sounds like your cup washer isn't very cup-y any more. You can try tightening the screw, but sometimes the washers are just done. Cup washers are fairly ordinary, so if you've got it, take it to the hardware store and get one just like it. If you don't--and sometimes they break, and you lose the pieces--take the screw out, take it to the hardware store, and find a cup washer that will fit.
Important! When you put it in, make sure the dome side of the washer is facing you! That's what will give it tension.