Grrrrrrrrr!!!

[Lance]

I just bought two Sure Grip 50 Powerhead Holders for my Maxi-Jet 1200's w/ the 1600 Upgrade kit. The ad on J/L says works on glass up to 1/2" thick. The box says: "For up to and including 1/2" aquarium walls. My tank has 1/2" glass: I'm all set, right?
So I place the PH's where I want them and plug them in. First they start to slide on the glass and then the magnet on the inside of the glass lets go and the PH tips away from the magnet. "WTF" If I push them back on while the pump is running, they seem to stay put. However, I want to run these on a controller for a little wave action. I'm sure as hell not going to stand there and manually replace the magnet everytime the power goes on.
Anybody else have this problem with these PH holders?