Replacement Screen isnt touching after repair

The customer brought in a an Iphone 6s Plus for a screen replacement. Before we do any work we thoroughly run the phone through a series of tests so the customer can see that everything is working on the phone before the repair(test phone call, volume buttons, home button etc..). Everything checked out except the phone wouldn't take a charge. This also helps us Identify anything that isn't working before we do the repair so the customer knows we didn't break it during the repair. So after we checked it in we replaced the screen and everything would come up but it would not touch and still would not touch after three different NEW screen replacements we tried. We also put the customers old screen back in and it will not even touch. What would be causing this? Touch IC Chip?

Any ideas would be greatly appreciated. Thanks