Hi Cinu,
Are you using any third party software's in between SAP & the tape lib itself ? If yes try to check with the scripts of the software as scripting plays role to up your database even when you're performing offline backups (Best e.g for the scenario are Veritas Netbackup or Net vault).
One more thing here i would like to check with same scheduling of backup on disk instead of tape drives.
Regards,
