Also known as Touch Disease, the iPhone 6 and 6 Plus have a logic board that does not get adequate support and the underfill seems to be insufficient too. This means that the phone’s touchscreen ends up failing or does not work effectively after some use. Users have noticed a gray flickering bar at the top or unresponsive screens. The problem occurs when the chip that powers the touchscreen is dislodged from the logic board. The problem can be solved only by a professional technician by replacing the logic board or by soldering the loose connections.
Solution: Take the Phone to Apple Store
If the phone is under warranty then you should definitely look for help from Apple Support. Apple has started a paid repair program for people facing the problem with the phone’s touch. If your phone is not under warranty then you can consider taking it to a professional technician for help.