I expect my screen to go inactive when I hold my iPhone 6 up to my face. This is not happening, and I'm hitting mute with my cheek.
The proximity sensor appears to be working, at least according to the three "phone test" apps I tried, all of which said it triggered when I moved my hand toward it.
Assuming the hardware is working, what else might cause this? I am running the recent version of iOS 11, and do not have a screen protector.