Numark N4 Cue Buttons Don't Work

Hopefully someone who uses a Numark N4 controller can help. The headphone "Cue" buttons above each channel on this controller aren't working. I can't get any audio into the headphones for finding cue points in a track while mixing. The "Cue/Main" mix knob at the top of the controller appears to be working fine. I get all of the main mix through the headphones when the dial knob is turned all of the way to the right - but nothing when the dial is turned all of the way left when a cue button is pushed above a channel fader. The controller cue buttons work fine with Serato and Virtual DJ. However, Mixvibes Cross is vastly superior software so am hoping someone has an answer - mapping solution.