Among other things, I believe your tape is rewinding after each use. If you use /dev/st0n, which is the non-rewinding device (n), then it will sit where it is left and continue on from there.
You indicate "we" and "servers" in plural. At some point, it can become a pain managing backups of multiple servers with your own individual commands and scripts. Have you considered looking at some sort of backup management software? My personal suggestion would be Amanda. You can find links here: http://www.linuxquestions.org/bookmarks/tags/backup