I have this same problem, and have tried updating just about everything (drivers & BE), with no resolution.
In my case, the drive (132T) is connected to a Qlogic HBA (fibre channel card). It (and the IBM Ultrium LTO-2 drive inside) are seen by BE, and the library can be inventoried, etc. but just won't seem to mount any tapes on the drive.
When I ran the job monitor, I noticed error messages in the form of "invalid handle" (for a vey long device string, starting with "/.//tape")
Any thing else I can try?