try to run this command to see what media are occupied.
to check if You do have scratch media run this
vmpool -listscratch

vmquery -b -pn SCRATCH_POOL_ID

where SCRATCH_POOL_ID is the output from the first command
depending on the robot You do have check for the column robot type (TLD or ACS or THM etc) to see how many of these are in robot.

You may find that you have plenty of media that NetBackup could use but may be frozen. Check to see if there are any tht exist nd determine why they were frozen. Perhaps you can simply unfreeze them and not mess around with production images.

DOCUMENTATION: How to troubleshoot frozen media on UNIX and Windows

http://support.veritas.com/docs/249632

Have a look into %install_path%\VERITAS\NetBackup\Bin\goodies
available_media script usually sits there