Camera not working when back is on.

Last week the focus motor on my HTC one gave up so i ordered a new unit and have put it in. I put the phone back together and then tested it before putting the metal case back on. It worked, great. Powered the phone down and then fully re assembled it. Started it up again to try the camera and nothing, get a black screen in the camera app and then it crashes after a few seconds. Long story short this problem only occurs when the phone is in the metal case. I figured it out by having the camera app open whiles putting the case back on, when im clipping the top of the phone in, the app freezes and crashes, if i pull the top out before it crashes then it continues to work fine. I am really at a loss with what to do. I only want the rear camera working as i need it for college work.