Unable to read unused QIC-40 tapes

A few days ago I finally managed to find and fit a perfect replacement for the pinch roller in my apple 40Mb tape drive. The old roller, like most these days had become a sticky mess and fitting the new one took a half hour with a dremel and a bit of magic with a freezer so that the replacement slipped on and would never come off once it warmed up. Well after watching the thing properly retension a spare QIC-80 tape I had (I have loads of QIC-80 tapes but only a few QIC-40 tapes) I then grabbed an unopened copy of A/UX 1.1 I had been wanting to backup and popped it into the drive and carefully watched it do it's initial retension (like it does whenever a tape is inserted). After a few moments I started to see the tape was not properly spooling back up (it was starting to move up and down on the spool) and I pulled the tape before it ate itself (or at least mashed up the sides of the tape). I fed it another QIC-40 tape and it did it again so I tried the QIC-80 tape again and it didn't happen. The only difference between my QIC-40 tapes and that QIC-80 tape is that the QIC-40 tapes have never been used. Could it be that the drive belts in the tapes are not enjoying me trying to use them after at least 20 years of no use? I won't even risk backing the tape up until I can properly get it to run through the retension cycle properly. More info on the Apple Tape Backup 40SC can be found here: http://docs.info.apple.com/article.html?artnum=704&coll=ap John.