There are 2 ways media ends up being retired in Backup Exec
A) You decide to manually retire the media. This method leaves catalogs etc on the system all it does is remove the media from being writable by any further jobs
B) Backup exec detects a problem with reading or writing, to or from the media and moves it to retired for you. This is done when the error detected means it wiould either be impossible or dangerous to continue using the media. (this appear to be what is happending to you.)
There are a number of things that may cause B
1) You wrote to the media with a different block size than your current setup is using. As I believe our defaults use the same block size for the same types of drives, this would have been a deliberate change to the normal settings and perhaps done for performance reasons. The fix for this is configure the current environment with the original block size (but that of course will affect any media written in the current environment too)
2) The drive itself is dirty and needs cleaning
3) The current drive has a problem. If you have another drive you could see if an inventory works in the other drive, it may be worth using a different tape (one that you do not care about the data on it) to run some of the hardware vendors tape tests against to get the drive checked. Also look in the event logs for hardware issues around the time you are trying to inventory the tape as these may provide more info on what is happening.
4) The current drive is incompatible with the type of tape - i.e. typically LTO can read 2 levels back and write 1 level back. As such an LTO-6 drive probably cannot read an LTO-3 tape even though the tape will fit in the drive (fix for this is get an older lower level drive). As you are using LTO-6 this is unlikely however, note an LTO-5 drive cannot read or write from LTO-6 so you cannot go down a level either.
5) The tape itself is corrupt (to check for this try a different tape from a similar timeframe and see what it does (also check a recent tape with inventrory, catalog and a test restore operations)