Melanie,
Dont despare, I have had this problem with nearly all versions of Netbackup.
I would re-inventory the robot from the GUI, update volume configuration then search for tape thats driving you nuts, click on it and try to delete it. IF an error comes back like a 92 then the tape has valid data on it.
Everytime I see this I mark it down on a sticky pad note thingee and go have some coffee and forget about it, then in a few days I try to expire it again and it woks.
JD