If you look at the bottom of a standard 880k floppy disk, you will see a small square hole in the middle of the disk (bottom metal bit) and beside this, at an angle outwards a rectangular hole in the metal, this is the hole the drive inserts a rotational pin into to spin the actual floppy insert inside the 3 1/2 inch plastic.
I'm probably not making much sense, just flip over any floppy disk and you will see what I mean.
On some A500's the spring loaded drive pin gets encrusted in dust/dirt/cat hair/fluff/dandruff/whatever and will not engage into the disk slot, therefore cannot rotate the disk reliably at the correct rotational speed.. and therefore the drive cannot "read" any disk. You do hear a slipping click sound from the drive, and an occassional disk will work, which is the symptom for this fault.
So take a cottom bud, soak in alcohol/spirits and clean the engage pin (rotate the drive so its closest to the slot, for easier access) its visible when you look inside the drive. Clean it as best you can, then try with a Workbench disk.
best of luck
