I have noticed this with customers that do not have a very good inventory of backup tapes, and rotate the same couple of tapes in and out of the tape drive constantly.
It is also a good idea to periodically clean the tape heads.
I prefer to eliminate the tape backup altogether, and simply backup to a Network drive. I do a weekly full system backup, and daily user archives for all of my CallPilot servers, and I have never had a problem restoring data from the network.
I regularly get users that have accidentally deleted messages that they want to get back, and as long as they were backed up, I have never had a problem restoring from the user archive.
-just my 2 cents.