Server Backup on 4mm Data Cartridge

This may sound like a stupid question, but this problem is new to me. Everynight we backup old school to 4mm data cartridge tapes. I clean the heads and purchase new tapes every few months, but suddenly two days ago we can not run backup, everytime we start the screen just pops up saying ‘tape rewinding’. It stays that way for hours if you let it, it will never run the backup. I have checked all processes, run a scan, a defrag, rebooted, changed tapes, etc..nothing seems to stop it. Im not sure what to do, my system has to get backed up.