Plenty of factors in tampering can cause this issue, and the most frequent are listed below. Follow the diagnostic steps below to figure out where the failure is.
1) The Connections to the screen are loose: open up the device, re-remove the screen, and attach it again. make sure you hear a click with each connector you apply. Should you have access to it, i would also suggest cleaning the connections with 99% isopropyl for a longer lasting device.
2) The Flex Ribbon has been damaged: scan the flex ribbons for any physical nicks or kinks. if you find anything, replacing the screen should fix the issue.
3) Touch Controller IC: Low quality screens, lack of Antistatic protection, and use of metal tools can cause static discharge which damage the touch controller. A much more advanced repair, replacing this IC should fix the issue.
4) Missing/damaged components in the related circuit: There are Hundreds of components on a board. using the wrong tool in the wrong spot, slipping with the drivers or even the original drop, can cause failures on the touch circuit. Manually scan the board for damaged or missing components in the areas you have worked, and replace as necessary.
5)Lemon Part: You may have been shipped a problematic part. in which case, your supplier's warranty should cover the problem, as long as you have tested the part before full installation.
6) Motherboard damage: If ALL else fail, you have a perminant failure in the PLB, and need to replace it.