5 x iPhone 4S repairs and proximity sensor fail every time?

Hello!

Basically I've completed 5 iPhone 4S repairs and every single time, the repair has been successful although the proximity sensor doesn't function on any phone.

4 of the repairs were changing the damaged screen, and the other one was simply replacing the flex cable. The flex cables are genuine replacement parts, the screens are of very good quality but not oem.

I've tried many different methods after researching online, yet to no avail, these include:

Sharpie method

Black electrical tape around prox and ambient light sensor

Tape with sharpie

Changing flex cable

Is there some method that I should be trying, I wouldn't be so bemused but 5 phones is basically telling me I'm missing something. I carry out the repairs with precision and have completed around 50 repairs with no problems prior to this.

Any help or advice would be so greatly appreciated as this is driving me stir crazy now as you can imagine!!

Thank you to anybody who can shed some light on this situation.