Screen replacement work fine then randomly becomes unresponsive. Why?

I replaced two screens, one on a i phone 7 plus and i phone 6 plus. They both worked fine for a couple of days after being replaced. Then issues started to occur with the screen. You will try to select an app and it would randomly open a completely different app that you didnt select. When the screen is locked and you attempt to open it with the passcode it will select numbers you didnt press. Completely random. Sometimes it works fine and then just starts to do whatever it wants to. Can anyone please help?