Nick
I would suggest that you check the tapes you are using. It sounds like you may be using AIT-3 tapes in your AIT-4 drive.
I have an AIT-3 tape library and an AIT-2 standalone drive both from Cybernetics. Backup exec has never had any capacity issues, although it has never reported compression accurately it has always worked right with my hardware. Most importantly, I can tell BEWS that I put in an AIT-2 tape, but if it is really an AIT-3 tape in the drive, it will STILL put 100 gigs on it. Backup exec does not 'decide' the tape is full, the drive tells Backup exec when the tape is full - it is a hardware function.
Check your tapes - they should be labelled "SDX4-200C" if they are the right tapes. if not then you have your problem. While your AIT-4 drive is backwards compatible with all previous generations of AIT tapes, 1,2 or 3, it will only get the capacity of whatever type of tape you put in it, ie 35, 50, or 100GB.
If this is not it, the next place I would be looking is OS drivers. The 'tape is full' function is just not something BEWS performs.