Got exactly the same problem with a Lumia 735. Reason was that I was following the wrong YouTube manual which did not tell that 2 pieces had to be moved from old to new screen: next to the bigger rectangular piece you also have to replace the smaller one including the attached small circuit sheet.
Check out the video from Crocfix: "Nokia Lumia 730 735 LCD Display Screen replacement by CrocFIX".
Works like a charm now and hoping to use this great phone for another 2 or 3 years.