When he pressed on the valve flap using his finger on the female QC, the hiss of the gas was unmistakable. No doubt that there's gas pressure in the hose.Worth checking to see if the propane female and the NG female are alike.
But, where is the NG going that the OP is hearing? I think that is at the top of the list right now.
Now, when the male QC (generator) is inserted into the female QC (hose), there's a bar protruding on the male QC that should push into the valve flap to allow gas through. However unlikely, the male protrusion may not be reaching the valve.
Just putting this out there, especially if he's not smelling any unburned NG coming out of the exhaust.