@Rob > Sorry to hear about that, I see the ticket went in on Monday but not sure why the Europe team never picked it up -- let's see if we can get you sorted out here.
Okay, for starters, are you using the CDJs to play content on a USB drive exported from rekordbox? Or are you connecting them via USB cable to a computer as a controller?
Next, does the player assignment on the CDJ / DJS match the channel to which it's connected on the mixer?