Backup exec 15 random stop detecting some of the tape on the drives

I am running backup exec 15 on Powervault TL2000 with 24 tapes. The program is having issue detecting some of the tapes. For example, slots 9 cannot be detected. I have switch a good tape into slots 9, but still cannot. I run the inventory on the tape library, hope to detect the tape. But it did not work. The program just cannot detect tape in the drive. Could advice what other stuff can I do?