In order to troubleshoot the problem, kindly refer the following technote link.
How to disable, delete, and then again re-enable a device in Backup Exec 9.x
http://seer.support.veritas.com/docs/268626.htm
How to recreate tape drives and autoloaders in VERITAS Backup Exec (tm) for Windows Servers
http://seer.support.veritas.com/docs/192144.htm
Also, Uninstall the existing device drivers and reinstall the latest Veritas drivers. Run tapeinst.exe to install Veritas driver.
"TAPEINST.EXE" is located in the directory x:\Program Files\VERITAS\Backup Exec\NT (TAPEINST.EXE" is an automated installation application for installing tape device drivers )
http://seer.support.veritas.com/docs/199881.htm
VERITAS Backup Exec (tm) 9.1 - Device Driver Installer (20050115) - Includes Tape Driver Release (20041231) and Robotic Library Release (45) (Intel Only)
http://seer.support.veritas.com/docs/273853.htm
We hope this helps.