You need to run an audit occasionally to make sure that what TSM thinks is in your library, matches what is in the library.
The audit command is AUDIT LIBRARY library-name CHECKLABEL=BARCODE
The CHECKLABEL=BARCODE switch is optional, but it will make the audit go pretty fast, say 5 minutes. With that switch, all the audit involves is your robot scanning the barcode labels of all the tapes. Without that switch, the default action is to mount each tape, which will take a long time.
The audit may wait until all tapes are dismounted from drives, so it could take a while for an audit to start. Consider canceling tape processes if your audit is waiting, and you need it in a hurry.