Front camera randomly working

Hi everyone.

Today I got a second hand iPhone 6S,

It was listed as “battery and front camera not working”

I changed the battery. It made the phone turn on

Then I check the camera. As described it’s not working.

So I replace the front sensors assembly.

And now it’s still not working. I even tried swapping the entire screen and sensors with one from another 6S’s.

At one point of unplugging and checking the connection, I reconnected the cameras and the battery, then turned on the phone, and it worked !

But then I screwed back everything in place, it stop working again.

I suspect a faulty connection but what do you think ?

Thanks