hercules mk4/headphones

hello.i bought a used mk4 but: when i turn on the headphones at the front entrance i can only hear the music / sound that is coming out of the speakers and not the one that will play next.the knob "monitor cue to mix" is in the cue position and i press the headphone button on the console and nothing happens too.should i have any connection / configuration wrong or a damage in the mk4 ? thanks in advance for your help.best regards