What OS are you running?
I'd check to make sure compression is enabled at the OS level.
If you are running Solaris for example ensure you have the proper entries in your st.conf file.
Are you confident in the compressibility of your data?
Try using some "zip" application to see who much it compresses.
What is your tape technology?
David A. Chapa
Elovate
Managing Partner, CTO
Co-Author, Implementing Backup and Recovery