Please make sure that while you select partition the autoloader, ensure that tapes are present in the slots so that when you fire job to the partitions there are tapes present for you to carry out the job.
Kindly refer the following technote as they would surely help you.
a) Title:- Creating, configuring, and targeting a backup job to a Robotic Library partition in VERITAS Backup Exec (tm) 9.x for Windows Servers
http://seer.support.veritas.com/docs/262055.htm
b) Title:- How to partition a loader and still be able to target a specific drive to be used for a backup operation
http://seer.support.veritas.com/docs/241773.htm
We hope this helps.