Hello, Santosh87... A few things you can check:
1) Check to make sure your tape and drive formats configured in TSM match what the tape really is. As stated above, TSM marks the tape full when it reaches an EOT marker regardless of how much data is actually on the tape. This kind of thing is typically seen when compression is enabled and something like a 2:1 or 3:1 is 'expected' but in reality it's less and you'll see a tape full well before the 'expected' capacity is written.
2) Check collocation settings and ensure things are set the way you think they are. I've seen instances where collocation is turned on and a single node or backupset gets written to a tape and then the tape is marked as full. Also check retention and reclamation settings.
3) If the tape is still in the library, do a move data to re-write the data onto a new tape and empty the existing tape and watch the percentage. It is also possible that you just have bad tapes - I've seen this too (especially back in the AIT2/3 days where it was not uncommon to get a whole box of bad tapes from Sony).
4) If you're really feeling daring you can delete the volume and backup the storage pool again which will recreate the missing data but only if you're feeling daring and have a full database backup and know that I'll deny any knowledge of this recommendation (but I've done it myself, many times).
Oh, do a q vol <vol_name> f=d and see if that shows anything of interest as well.
-b