Are you saving them directly onto the disc as Excel files, or are you "zipping" them and saving the zip files to floppy?
If the letter then when you "open" them you may be opening a version of the file that has been extracted to a temp folder. Unless you resave this into the zip/archive file I would expect the symptons you are experiencing.
If no zipping is involved then I'm at a loss. It sounds as if it's a "saving" problem, but I'm afraid I have no answer!