Issues with Proximity Sensor

I have replaced the screen to a number of iPhone 5S and I never met this issue before. The screen replacement goes smooth and the camera, light and proximity sensor cable is perfectly seated in its place . I transferred the little white filter from the broken screen because this is a black screen.

Proximity sensor was working and not it is not working anymore. Reopen, triple check the seating of the sensor. Close. Nothing.

Then tried to replace the whole flexy. New cable, new sensors. Nothing.

I dont really know what to do anymore. Hope you guys have some hints?

Many thanks