You mean you keep the paper backing on the UTH? That's probably why you're having problems. It's probably best if you stick the UTH directly on the glass, because it'll be more efficient with heating and you won't have to run it as high. If you want to be able to un-stick it and reuse it on another enclosure, what you can do is remove the paper backing and then use aluminum foil tape (shiny side out) to cover the sticky side so it's not sticky anymore, and then with some more aluminum tape you can tape it onto the enclosure. That way you can untape it whenever you want.