LCD won't work when installed, but works outside phone

I have an extremely weird problem and one that has had me going at this for over 6 hours combined.

I'd first like to say I have done over 200 or so iPhone's so I know whats going on. Except this time.

I first tried to replace the LCD on a cracked 4s and everything went smoothly until I tried turning it back on, nothing happened... i thought great.. defective LCD, so I went ahead, grabbed another, tested it with the phone before I installed, saw that it worked and switched them out.

As soon as I tried turning the phone on again, nothing happened. Confused and not really knowing what was going on, I made sure all the connections were strong and tried again, nothing. So I just randomly tested that first LCD (in the same phone that I couldn't turn on and it did turn on and worked normally)

Again, I switched out the LCD's to make sure I didn't screw anything up (call me insane) and again, it doesn't work. I can plug any screen into the phone just by connecting the two LCD connectors and it will work fine. The trouble is when I try to connect the cables from the LCD that is already installed.

Things I've already noted:

At first I thought it might have been shorting so I covered the back of the board with electrical tape (only the part that touches the grey LCD box type thing on the connector) - nothing

all cables are secure

Ive also tried putting the logic board into an entirely different iPhone and it worked once, but of course the only one it worked on was a cracked screen, when I replaced that one, nothing.

Please if someone could help me out, it would be greatly appreciated