first question is, did this tape drive work before with the full capacity? if so then it could be that BE is not using compression. go into your preferences and check the tape drive setup to make sure that BE is allowed to use compression. if the tape is 400 GB compressed then it is 200GB native and that is the maximum that it can use without compression.
if it did not work before and it is a new drive then I would still check that setting. then check with BE tech support to see if that drive is supported by them. if so you might need a firmware update for the drive.
as far as backup practices, we have moved away from Backup Exec to StorServer. BE is a good job but it doesn’t compare to StorServer as far as backup schemes. Storserver does one full backup and from then on it only backs up the data that has changed. it backs up to disk natively and then creates disaster recovery tapes from those backups. so normal restores are done from disk, (accidental deletions, etc.), and the only time that you go to tapes is for disaster recovery. it is harder to understand but once you get used to it the product is great.