Problem with connecting DDJ 400 to amplifier

Hi, I don't do a lot of private gigs but lately had a couple of bookings and I just take my DDJ 400 and Macbook to connect up to the venue's PA/sound system. I've had problems with the connectivity and sound, however, one thing I note is the cue function disables in my headphones. Not entirely, but almost, around 75/80% of the sound. I'm fully aware of the controls and cue levels on the DDJ 400 to get more sound through the headphones but nothing works. Only if I'm lucky enough to have the master song playing with low moment at the end of the song (no vocals, mids just a quieter base line) am I able to 'just' hear the cued song in my headphones. This has now happened on two occasions, and I end up cueing the next song by ear. Not bad to have to learn to do but not ideal either. Any help on this matter would be greatly appreciated.