Along with the two previous recommendations, you can determine what the OS sees and compare it to what NetBackup sees for the library. From the connected media server, run:
/usr/openv/volmgr/bin/robtest
Select your problematic library
Run: s d
This will Show you the tapes in the Drives (s and d) - make note of the home slot for the media
From the NBU Admin Console, go to Device Monitor and look at the drives of the problematic robot. If the tapes in the robot match what was shown by robtest, you can check to make sure that the slot they're supposed to go back to aren't occupied.
From your media server running robtest, use: s s (this will Show you the tapes in the Slots)
Make sure that there aren't any tapes in the home slots for the tapes in the drives. If there are tapes in the slots, the robot can't return the tapes to their home slot, and the tape will be 'stuck' and the drive will be marked down. You can confirm this by looking at the NBU Admin Console > Media > sort by "Slot" > see if NetBackup matches what robtest showed.
If you see that there are tapes in the home slots for the tapes in the drive(s), you can manually move the tapes around using robtest to free up the slots so the robot can unload the tapes.
If you need to use robtest to move tapes, use: m s[slot number] s[empty slot] (This Moves the tape in Slot[1] to Slot[2].
Do this for tapes in the home slot of the stuck tape, then you can move the tape from the drive to its home slot. To do this, use: m d[affected drive] s[home slot]
This will return the tape to its appropriate home slot. Confirm that the tape has moved to out of the drive (s d from robtest) and to its home slot (s s).
Once that's done, exit robtest (be sure to fully exit, as the library will be unusable if robtest is still running).
Go to your Admin Console > Devices > Robots > Inventory Robot... > Preview Device Configuration Changes and see if NetBackup registers the moved tapes.
If the inventory moves the tapes, your library and NBU should be synced back up and the drive should return to functionality, assuming there aren't other underlying hardware issues.