Dell PV132T Tape Backup Library

I have Windows 2003 Server with Symantec Backup Exec 12.5 for Windows Servers and Dell PV132T Tape Backup Library with Two SDLT Drives. Simultaneous back is not happening when i schedule the job on both the drives and I have updated the drivers, firmware and tried all my options. i am able to make the drive online when it going offline I have 19 tapes in the slots and 20th tape is the Cleaning Tape with the BAR CODE. I have cleaned the drive too. I am able to import or export the Media with no problems. Since the issue is critical expecting a reply ASAP.