* The 4 screws that hold the display down at the corners are not put in.
* The battery has started to expand putting pressure behind the screen.
* The housing of the iPad is bent in a way that does not let the digitizer sit flat without touching the screen.
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* Also there is usually a protective film that needs to be peeled off behind the digitizer which can also cause this problem as it adds thickness to the screen.
Usually it comes to one or more of the following:
* The 4 screws that hold the display down at the corners are not put in.
* The battery has started to expand putting pressure behind the screen.
* The housing of the iPad is bent in a way that does not let the digitizer sit flat without touching the screen.