Hello! I believe the issue you are probably having is a board issue where the components for the power button may have been broken off or damaged. In the image of the iPad 6 logic board below, the components I circled in red to the left go the power/volume button zif connector are the components that control the power and volume buttons. Depending on which of these get knocked off, one of the buttons may stop working. I've accidentally knocked these off when cleaning the adhesive off of the frame of the iPad 6 before, it is very easy to do, but no so easy to fix, the components will most likely need soldered back into place.