Gradual failure of iPhone 6 after screen replacement

I recently replaced an Iphone 6 screen and initially everything worked fine. The person I did it for was very happy. Two weeks later she told me that the touch screen had started to fail a couple of days after the replacement and had continued to deteriorate until it was very slow and hard to use. I had her do a hard reset (home and power button pressed simultaneously), then reset the phone and finally a full restore. I got no reply for a week and then she let me know that none of these measures had worked and that the phone had eventually stopped working altogether and wasn't connecting to her computer. (I can't confirm any of this as the phone was never again in my possesion) She had then rung Apple support and discussed the problem with someone who believes that I had damaged something inside the phone to cause this problem. Apple went onto to sell her a 'New' phone for $409 (more expensive because of the 3rd party screen) My question is - What could have been damaged to cause this problem? I didn't touch any components inside the phone other than to remove the shield and screen connectors.