It's hard to say, depends on the issue that made the iPhone malfunction in the first place. I've had problems with motherboards on the iPhone 6 and newer series, where people would try to turn the phone on before assembling the phone back together. By that I mean missing screws before the assembly, witch fried the motherboard and caused the iPhone's not to turn on, or to be stuck in a bootloop.
I wouldn't be surprised if the battery was making such problems. First thing that comes to mind is trying with a different battery. If that doesn't work, and a new charging port assembly doesn't help, you might be having issues with your motherboard.
Try prying it with a plastic spudger, starting with the bottom left corner of the iPhone's screen. It might take some more heating, and applying a certain amount of force, but not too much as you don't want to break it.
Dear Bill, I recently had a similar problem with an iPhone 5s (64GB model) where the LCD would sometimes cause bootloops. I would suggest trying with a different screen, even if yours is the original piece. Hope this helps. Regards
Dear Frímann, I've had a similar issue with the LCD ribbon on my iPhone 6. Problem was, I was trying to insert a bent ribbon witch was causing my LCD to die in a short time. I carefully reconnected the ribbon using a plastic spudger (took me a few times to get it right) and that fixed the problem, hope it helps you with the digitizer cable. Regards, Andrea
Sounds like a motherboard problem, at first. Does the volume up button click? If it does, I would take my chances and say it's a motherboard problem. Try installing it on another iPhone 6 and see if it works, but if you can't do that try with a third button's flex.