Know this is an old post but I was having the same problem after buying a replacement screen on Ebay. Turns out the front camera cover was touching the vibrate motor preventing it from turning. I fixed this by pressing the metal hook that is on the right side of the camera cover in a little bit with a screwdriver and now it's working. I also had previously tried repositioning the camera and camera holder but this didn't solve it. Let me know if this helps anyone.
