We have had this problem in the past. Backup was running just peachy for months then one night it would fail due to alck of space. We have the tapes set to Overwritable & Allow Append - Infinite. We are writing approx 24BG to the tapes which is then compressed about 1.30:1 to about 16GB. The only way to get the backups working when this happens is to reinstall the software. I am not sure why the backup suddenly "turns off" the compression, or why we cant turn it back on again.
Id love to see if someone has found a fix other that a reinstall of BE 9.1.
thanks,
Dave