Headphone cue not working on DDJ SB

Hey, 

I bought a Pioneer DDJ SB controller last year or the year before and it's been fine upto my last gig.

Basically I'm not getting any audio when the headphone mix knob is on cue and headphone level on max, also when I have a deck cue'd the volume on the master output seems to amplify when the headphone mix is on cue, but surely the headphone mix shouldn't affect the master volume?
Please advise, thanks