Proximity sensor not working after Power button replacement.

Hello,

I am an Iphone tech in and am working on a CMDA (Verizon) White iPhone 4. the power button was broken and did a replacement on it. the new power button works perfectly but now the new proximity sensor is not working at all, ive tried several replacements and seem to be getting the same problem from each of the new replacements. knowing that 5 different, brand new, flex cables can not all be broken i feel it has something to do with the iPhone itself as the original proximity sensor does not work whatsoever either. i'm not sure if it has to deal with the software not recognizing different ones as i have done many replacements and never run into this problem before. i was hoping to get input as i am at a loss of what the next step could be. please help me as i do not want an unsatisfied customer and want to be able to fix any problem i come across and this problem has me stumped. also everything else on the phone is original so this has me even more dumbfounded. Hope someone can help me