Try a new screen it could just be a defective display, Ive seen quite a few defective displays in the repairs I've done, get them from Amazon or iFixit, i have found eBay has a lot of sellers that send defective parts, Ive found the best quality ones you can get are when I have bought them directly from China. Hope this Helps
Hi, Order a new display and try fitting it, I have found also if you muddle up the screws that hold the metal plate that covers the connectors in the wrong place it can sometimes muck around with the screen
Hi, I would recommennd getting some 70% isopropyl alcohol wipes and remove the logic board and clean all the connectors with the wipes, if this doesn't work try and fit a new display there is a brilliant guide on this website, could you upload a picture of what the device screen looks like? hope this helps?