XDJ XZ not playing through Deck 3 using Rekordbox

I have Decks 3 and 4 selected and Ive loaded tracks into ALL decks.
Deck 1, 2 and 4 are playing fine but although a track is loaded into Deck 3 and essentially "plays", the waveform is greyed out in Rekordbox and Im getting no sound output and on the BPM display its saying 'EXTERNAL INPUT' which suggests a setting is incorrect somewhere?
Ive looked through the Preferences but cant see where I can change anything?
Any ideas?