Backup space and compresseion

Quantum dlt 4000 backup tape drive with 40gig dlt tapeIV for media. According to the job log, the weekend backup is looking for more media to get loaded to complete the backup. It is stopping around 45gig. We have hardware compression enabled. The bytes processed do not indicate the space used on the tape. Is it time for a new tape drive? How can you determine which data is not able to be compressed? Some of it must be getting compressed if it stops processing at 45gig......am I overlooking something?