iPhone 6s Screen replacement goes black after replacement?

I have replaced a couple of screen but I haven't done a 6s. When I was replacing this phone I checked the quality of the new replacement screen, before attaching the camera and home button, by attaching the two connectors and powering it on. It worked fine so I went ahead and attached the camera and home button. When I reattached the connectors again and tested the screen one more time before closing it up, it was black. The phone was on because I could feel it vibrate, but the screen was black. I thought I may have done something wrong so I tried again a couple of times, but each time the same result. I thought maybe the screen was faulty so I tried another one with the exact same steps and got the exact same result. This has never happened to me before. Does anyone know why this is happening? Thanks.