Makes sound that it on but screen wont light up

I have replaced a good share of iPhone screen and a client brought me 2 iPhone 4's and I did the replacement on both of them i had tested the screens with a test phone and they work perfectly and when i finished the phones turn on and and are recognized by itunes but the screen will not light up at all. The phone vibrates and makes sound but will not power on and i tested the screens again and do light up but not when connected to the customers phone