swapping i9506 board into i9505´s frame

See if someone come through some light on my problem...

The screen of my i9506 completely broke, aswell as the external frame that was very damaged. I bought an i9505 which didnt even turn on and I swapped the i9506 board into the i9505 case, rather than changing screens.

It works good, but i find a strange problem.... the phone works flawlessly until i put the screws in the back cover to leave it sealed, when it turns off and there is no way to turn it on again until i completely remove the cover and frame, to manually press the power little switch in the inside. Only then the phone turns on again.

In case it was due to differences in the rest of parts between i9506 and i9505 or due to any other problem with any other part on the original i9505, i swapped all internal parts from i9506 into the i9505´s frame, excepting the frame itself with its screen, but the problem wasnt solved.

I hope I made sense discribing the issue im having, and someone can think of a reason why the phone turns off as soon as i put the screws in....¿?