Phone doesn't switch up correctly after closing rear case

Hi, i've changed the display of an LG G2 and after some difficulty in the re-assembling (depending of a malfunctioning of the new display) the phone works, but when I close the back cover the phone doesn't switch up... If I switch it up (without cover) and I close the back cover, the power switch works correctly, as well with cover closed, but if I switch it off I must open the back cover and press the power switch directly on the mainboard... I've try to change the flat cable which connects some devices to the mainboard (including the power switch) and also the power switch but the problem doesn't fix... What can I do? Thanks