Media issues on Full Backups

[Pokaroo]

I have been using BE 8.6 for about 2 years now, and generally have a good handle on it, except for media operations. I have full backups that run on Fridays. I have four Friday tapes, each Friday a new tape goes in and does a full backup. For about 2 months now these backups have requested a new tape be put in part way through the backup. The tape drive is a Compaq 40/80DLT, the tapes are 40/80 DLT from Compaq, the drive is cleaned once a month. Total backup size for full is about 33 Gb, takes about 4 hours. Compression is enabled on the tape drive and in the Options for the backup job. There is no error message, just says to insert overwriteable media after backing up approx 21 Gb. Overwrite period is set to 27 days and append to 45 days. I started erasing the Friday tapes before the backup a month ago, and they worked fine the first pass across the tape, now they are on the second pass (I can get 2 months worth of data on the tapes normally) and they are requesting another tape. Can't see what the problem is as there are no errors. Any ideas out there???