problema con sostituzione schermo iphone 8 plus

Hello everyone ,
Guys could you give me an idea about what the problem of my iphone 8 plus could be? practically I had the broken screen, but it turned on quietly, I decided to change the screen with a new AA grade compatible, testing it by attaching the two lcd and touch connectors and then batting the screen does not turn on, then attacking the old screen and trying again turns on, I’ve already tried two new screens but they don’t go, what can you tell me ??