Recording to Rekordbox from DJM750mk2 using USB link

Hi, I have connected my DJM750mk2 mixer to my macbook using a USB cable. I have updated all drivers, downloaded the utility software and put Rekordbox into performance mode. I have selected a file destination. I have followed forum advice and have set USB 1/2 in utility to Rec Out. I can see that my Macbook creates a new file for each recording in my chosen destination folder, however these files have no sound. Other files will play with volume (so it isn't a setting on the Macs audio). I now have 6 files with no audio. Master out volume is set to about halfway. What am I doing wrong?