Why is my new screen not working?

My at&t iPhone 4 screen is cracked and bough a AT&T front screen LCD digitizer unit all together as one unit and I took my iPhone apart and put the screen in and put it altogether didn't work I tried it 3 times didn't work so I put my old screen in and my old screen worked help me I don't know what the problem is I think my screen is a older one and the one I bought is newer or something help me please