Loose contact on LCD?

I've replaced the the entire screen of my sister's iPhone 6s. After that the screen didn't display anything while the phone would work as well as the Touch ID and the touch screen. After one day the display magicaly started to work but only for half an hour and then went black again (all other functions of the phone still seemed to work). one cycle (screen turning off and then turning back on again) could take from a few minutes to entire days.

Now for additional information: while replacing the screen I didn't disconnect the battery and during some attempts to finally fix the screen (I tried around 6 times reconnecting the connectors of the screen) also left the phone on. The touch functionality of the screen, the touch ID home button, the front camera and the ear piece (plus proximity sensor) all worked fine (all parts except the LCD were taken from the original screen). I brought the phone to a store and they tried a new screen as well as a new battery but none of them fixed the problem. The screen now still only sporadically displays anything.

My guess is the there is some loose contact of the screen connector socket and I've thought about trying to replace it but I haven't found a tutorial for that and neither am I sure whether that would actually help.

Does anyone have an idea what the problem is and how I can fix it?

Here is a picture of the screen connectors.

I've already inspected them relatively closely but they seem at least physically fine to me.