iPhone screen working weird

i have iphone 6s and i changed the screen. the screen is perfectly working fine on an other iphone but on this iphone after you hard reset it it works then it stops and when i shine a flashlight on it you see that the backlit doesnt work. after i reset it again the screen works again perfectly fine. then i put a new screen on the same iphone with the problem and it gives the same problem. what can i do?