Why doesn't the front facing camera work after a repair?

good morning! i have a problem with my iphone 4... I had to repair the display, because of a broken screen and when I closed, I realized that the front facing camera didn't work.. i was sure before the repair it worked so I tried to change, but I didn't solve it..

what it is strange, is that in another iphone they both worked. It never happened to me and I don't think I could blame on the mother board because I treated it well and careful.. do you have a suggest please? Thank you for making yourself available.