Phone not "sleeping" when I'm in-call

Up until recently, my phone's screen would go into sleep mode while I was talking, meaning the screen was completely black.

Now, the screen refuses to go into sleep mode and I find myself constantly hitting the mute or speaker button. I'm talking like 15 times/call, I'm about ready to throw my phone. It worked just fine for months until recently.

I checked all settings and I checked the manual and found no answers; does anyone know how to get my phone to go to sleep when I'm talking on it?