Same issue. I think it started for me with iOS12.1 and I still have it on 12.1.1 betas. It usually works after a reboot but then stops.
I only use Siri for a shortcut I have to translate what I say into another language. Weirdly, the setting to start translating after a pause works, but it sends nothing to be translated. In other words, it seems as if Siri is picking up the fact I was talking and had then stopped talking, but didn't actually transcribe anything I said. I've assumed it's a bug. it's been mentioned on here before I believe in one of the iOS12 beta threads.
I rebooted twice and still the same issue. Voice feed back is on, no settings are changed on my side. Just driving me nuts as I use voice prompt a lot while driving and hate to look at screen while driving
[doublepost=1560840713][/doublepost]Go Settings > Siri & Search, make sure the option "Listen for 'Hey Siri'" is switched on, it sounds like you may have inadvertently switched it off (this is a handy option if you have more than one Siri-equipped device in the same room, you can limit the response to the device you want to respond).