The iPhone 6 screen replacement repair proves to be a bit more fiddly than the previous 5 series phones. Although the screen ejects easier the problem lies in the awkwardness of the proximity sensor / front camera flex cable. The main issue I have come across is that when I replace the sensor onto a new screen the proximity sensor would not work. This issues is detailed on fixit here.
Previously I had just assumed the solder points on the sensor were coming loose when I had been removing the sensor from the glass panel or mounting. I had tried using the old iPhone 5 electrical tape and blacker marker tricks to no avail so assume it must be the cable.
The solution came to me about 2 months later, I had been putting these cables aside for training purposes and when I used this cable again for training technicians I went through a mock QC test however the proximity sensor did indeed work. We tried the old cables in all our new iPhone 6 screen repairs when we again had this issue and same result occurred. It appears the sensor is timing out but after a period of time (1 week +) it will work again.
We are only having this issue on white iPhone 6 phones about 50% of the time, so it could come down to some iPhone 6 phones having very sensitive cables. Since this solution we now switch all proximity sensor cables from old phones to new phones on white devices, never had an issue since.