My opinion is that the first reason for pushing up is the digitizer flat cable being folded and pinched between the glass and the aluminum frame.
If the digitizer is popping up first and foremost right above where the flat cable is situated, it means the above is true.
If the digitizer is getting loose right about every where, then you could be using the inadequate kind of double tape. I posted a link in the other review to the kind I've been using for the past year, and it holds everything perfectly in place.
Note that by following the exact steps in my review, I never have to use a book to weigh down on the digitizer. It just sits perfectly in place.
It could also be that the aluminum frame/case of the iPad is dented or bent (even slightly), not allowing the glass to fit in place.
I'll post later a picture of an iPad Mini that I'm repairing for you to see how I do it, in case it helps.