https://www.liveatc.net/search/?icao=kapaYou will see there is a Denver approach frequency listed in the feed description.
The Denver approach control will be switching to multicast at night: The controller will be broadcasting on several frequencies while the planes may remain on separate frequencies. So you could hear the controller talking to planes approaching DEN but the planes are actually on a different frequency.
Check the DEN departure feed for hearing DEN approach control at night i believe they use one of the departure frequencies.
You will probably then hear it's the same controller.
Regarding reception of aircraft:
Maybe feed reception is not good enough to pick up aircraft close to the ground or approaching from the wrong direction.
Have you heard the feed any better at some point?