My thought is the connector or ribbon cable. If the connecter is seated correctly move to the digitizer ribbon. I like to see happy cables. Nothing bent sharply or wrinkled. With the iPad turned on. You can hold the screen on its side. Dont put finger prints inside the digi screen…. While on its side carefully push on the ribbon cable. slightly adjust things and test the screen while its sideways. See if you can replicate it not working. If cables arent the problem it could be a bad digitizer. Also check to make sure nothing is touching the digitizer while its closed, a piece of glass, a screw not tight and touching back side of the screen. I have found all sorts of tiny crap that just barely touch the screen or ribbon and makes everything go crazy. Good luck. Troubleshooting iPads can aggravating because its always something simple or chasing a bad part.