I setup an entire AVR just for DD+. When I fired it up all I got was All Channel Stereo. I was pretty sad when I seen this thread. I was searching the interwebs and someone mentioned volume leveling.
I went into Vudu and played a movie that supports atmos (wonder woman). I hit the * button and turned off volume leveling. When the audio came back DD+ was enabled and working. Tested on Vudu, Disney, Netflix (didn't test Amazon, I assume its good now to).
Hope that helps.