Well the floppy controller could be shot. It might be able to read, but I consider it defective if it's not allowing you to write.
If this is on a new motherboard, look into getting it replaced as that's about the only way you'll get a working floppy drive. If it's older and out of warranty, you're basically stuck with: A.) A busted floppy port; or, B.) buying a new motherboard.