Unfortunatly floppy disks are not very reliable, this happens a lot. When left near anything that generates a magnetic field they can be corrupted. Things like mobile phones, monitors and tv's are just some of these.
To be sure it's the disks at fault try them in another floppy drive, perhaps you know someone who has one. It's possible the drive itself is faulty.
Have you tried a new floppy disk to see if it can be written to and read from?