Touch screen works when the screen is out of the phone but not in?

TL;DR: the touch screen works when it's just plugged in and not fully set down in the phone, but as soon as it is fully installed, the touch screen gets dead spots.

Hi, I work for a repair shop in Idaho, and I had a customer that came in needing his 5S screen replaced. He already had an aftermarket screen and button on it (I don't know if he did it himself or had some other shop do it), but it had cracked and the digitizer had stopped working. He dropped it off because I was busy, so when it was his turn I grabbed a new screen out of the back, plugged it into the Logic Board, tested it, and it worked great. So I transferred over his camera, earpiece, button, etc, put it back together and put it on the charger. He came back an hour later, and asked me to put his original home button back in, so I did. And as I started to ring him out, he noticed there was a huge dead spot that ran vertically all the way up and down on the right side of the screen. I thought it was super weird so I did a hard reboot, but that didn't help.

So I told him I would do some troubleshooting and that I would call him with what I found out. At first I thought it was maybe the proximity sensor because the screen worked before I transferred it over, so I unplugged it, tried it while the screen was out of the phone (it worked), and then tried it with the screen set in the phone (didn't work). That's when I noticed the pattern. So I grabbed the whole bin of like 7 or 8 5S screens and they all did that same thing. The connectors on the Logic Board look fine, there doesn't seem to be anything sharp that could be poking the ribbon cables when it's set in the phone, so what could it be??