Floppy drives can suffer from numerous ailments -- dirty/misaligned heads and balky mechanicals are the common ones. As Mike said, a good clean will often do wonders. Here are some troubleshooting tips:
1) If a drive can successfully read a disk that the same drive also formatted, but it cannot read disks formatted by a different Mac, then the problem is likely a misaligned head.
2) If a drive cannot read a disk formatted by any Mac, but that disk is known to be readable by other Macs, then dirty heads or some mechanical problem is likely the reason. Clean the heads first. If that doesn't solve the problem, you'll have to get at the drive and disassemble it. You'll likely find that the chassis lubricant has morphed into a glue-like substance. Carefully and thoroughly remove it (WD-40 on a cotton swab works well), then re-lube (don't overdo it).
A thorough head and chassis clean will fix most of the common floppy drive problems.
1) If a drive can successfully read a disk that the same drive also formatted, but it cannot read disks formatted by a different Mac, then the problem is likely a misaligned head.
2) If a drive cannot read a disk formatted by any Mac, but that disk is known to be readable by other Macs, then dirty heads or some mechanical problem is likely the reason. Clean the heads first. If that doesn't solve the problem, you'll have to get at the drive and disassemble it. You'll likely find that the chassis lubricant has morphed into a glue-like substance. Carefully and thoroughly remove it (WD-40 on a cotton swab works well), then re-lube (don't overdo it).
A thorough head and chassis clean will fix most of the common floppy drive problems.