Seems like a sound routing issue.
On Traktor, check your output routing in the settings. You should have 4 outputs. 2 for master and 2 more for monitor.
Can you please play a track on Deck A with the volume right down, the monitor cue button on and the monitor mix knob all the way over to cue … if you can’t hear a track then your routings are wrong.
Sorry I can see how my comments are contradicting.
The knob increases/decreases the amount of cue you can here mixed in with the master but it won’t increase/decrease the amount of master mixed in with the cue (if that makes sense)
Every other DJ mixer/controller I have ever used has the cue playing in its own and then the headphone mix knob brings in the master to allow you to hear both mixed in the headphones where as the DDJ is the complete opposite and you can’t hear the cue on its own