iPhone 5S front facing camera freezes when switched from rear to front

Alright, I'm a Cell Phone Technician and I work on mainly Apple stuff, thats what I prefer to work on versus some other phones that we work on. I have good experience and have been doing this awhile so before anyone starts the insults I would appreciate if you didn't. Just wondering, my co worker had done an iPhone 5S and when they brought it in for the screen replacement he had changed out the screen and put a new one in. After market of course, but not the front facing camera isn't working. The app opens, the rear facing camera is working as well, the only problem that occurs is when you want to switch to the front facing camera from the rear camera. Anyone know what could have caused this or have an idea? It got bumped into my lap because I have a bit more experience than him and I can't figure it out. I've also replaced the original front facing camera with a whole new flex and proximity cable. Let me know! Thanks guys!