headphone cue triggers play through master even when crossfader fully to other deck and channel volume is down

I 'm a youth worker and bought a  pioneer ddj 400 which has been working fine for a couple of months. Was trying to teach a young lad how it works today and the headphones cue now triggers the track through the master and not the headphones. 

I'm totally bamboozled- sure it must be a setting has been altered but don't know how to access option and what to look for?