Camera issue-shutter does not open

I have been buying and repairing Iphone 4's mostly Verizon CDMA and Iphone 4S's. I have noticed an issue on many of these phones with the camera that is not fixed by replacing the camera itself. Most of the phones with this issue have been the Iphone 4 Verizon but I just found an Iphone 4S with the same issue. When trying to start the camera function the shutter stays closed. Sometimes only the front or back camera will work but for the most part neither camera will function. I have spent hours trying to solve this common issue. Occasionally it is cause by a bad front or back camera but usually it is something in the logic board. I have moved logic boards to a different phones and the problems persists. I have come to the conclusion that there is no solutions. I have tried everything in the world including changing cameras resetting the phone and every other solution suggested and nothing works. I have to accept there is no solution. Everyone with this problem usually winds up going to Apple and replacing the phone. If anyone has any new solutions that might work I would greatly appreciate it.