Floppy drives suffer many indignities at the hands of users, and at the hands of time.

First try cleaning the drive (yes, you'll have to open up the Mac to do a proper job). Gently clean the heads. Make sure that dust and/or gummed-up lubricant is not inhibiting moving parts from doing what they're supposed to do. In particular, verify that the top head really does make contact with the disk surface, rather than stopping some distance above it.

Those are first steps.