DJM-A9 - Headphones A - No sound when turn to Master

Hi, Can someone please help me figure out why I cannot get any sound when I turn Cue-Mix-Master knob to Master? Only time I can here anything in headphones is when the channels Cue button is activated. It, however, does not let me isolate the two channels by selecting Cue or Master. I am coming from Denon DJ equipment and like to listen to whether songs will mix together within headphones. Hopefully what I am asking makes sense. Please advise. Thanks.