How many slots does your library have in total? Is it 40 or more than that? If so does the Backup Exec GUI show slot 40 within Device Management, or only go up to 39?
We've got a 20 slot autoloader, however the 20th slot is technically designated as a mail slot, so even though I can physically put a tape into it, it doesn't get displayed within Backup Exec, and can't be inventoried. So I was just wondering if your situation could be the same. Have you tried doing an inventory of any other individual slot?