Sounds to me like a hardware problem. If you are sure that the tabs are in the right position, then I assume that the sensor inside the floppy drive is bent, misaligned or broken. Have you got another PC you could "borrow" a drive from and try?
Is floppy the only way you can transfer these files? It is a very old-hat way of doing things, these days. Have you considered eMail, FTP etc as easier ways to do it?
Timothy