iPhone won't turn on at all.

I replaced a screen on an iPhone 6. I've been doing this on many devices and everything was going smoothly. I was missing some components so I bought two replacement screens with all of the components for the screen. I installed the original home button onto the screen and placed the ribbon cables neatly and screwed the proper screws in the different holes. I turned the screen on but it wasn't working. Dang. A bad product... I tried the other screen with the phone's original home button and placed the ribbon cables and screwed the screws. Now it worked! I placed the working screen on a different iphone and let this one sit for two days. I installed the home button for the phone in the original screen since I were going to put the phone in DFU mode but the *!&$!&$^ thing doesn't start now... I've tried with two different batteries, three different (shattered) screens but still no luck. I've plugged it to my pc but itunes doesn't recognise it... Please help me! I have limited resources but I will try anything.