Sound then no sound


Running Audacity 2.2.2
Mac Osx High Sierra 10.13.6

I am trying to digitise some of my old vinyl, the equipment I am using is as follows

Pioneer turntable
Pioneer DJM900 NXS Mixer (has built in 24bit A/D Converter)
MacBook Pro (retina, no line out) connected via usb

So I connect everything up and sometimes it seems to work and then other times audacity does not register any sound at all. Music is playing and I get sound through the speakers. I have managed to copy a few tracks but when I went back to try some more I get nothing. I have tried turning everything on and off, unplugging and plugging back in. I cannot for the life of me think why this is happening, any suggestions?