[OPEN] CDJs are playing but no signal to the mixer and no sound from speakers...Please Help!

The problem I’m having is a technical one, and if I’m honest its really starting to pee me off :) I’m no technical noob by any stretch, but I’m really stuck on this one.....I will try and explain the best way I can, so bear with me...

So I have my setup ready to go (3 CDJ 2000 Nexus & 1 DJM 900 Nexus) RCA cables running from each CDJ to the mixer, speakers and sub running from the mixer – I have a playlist exported from RB on a USB stick plugged in, and each CDJ is synced via Ethernet cables with a switching hub and is connected fine as I can see the playlist on each CDJ.

 CDJ’s are assigned player 2,3 & 4 in channels 2,3 & 4 to the mixer, when I play a track from CDJ 2 & 3 the CDJs are running and playing the track selected but there is no signal to the mixer, but when I play the same track in CDJ 4 through channel 4 where is a signal on the mixer and the track will play through the speakers.....

 So I take the RCA cable from CDJ 4 and plug into channel 2 &3 in the mixer (the channels where the other CDJs won’t play properly) with CDJ 4 still playing and the track plays fine through the speakers and the mixer as a sinal, so I take it I have eliminated the mixer being U/S – I’ve also tried different RCA cables and they work fine for player 4 but not for 2 & 3?????

 So could there be something wrong with the 2 CDJs (2 & 3)? Or a setting somewhere I’m not aware of? They all work fine with Traktor, but its seems as soon as I plug RCA cables in and try and run standalone they will not produce sound through the mixer or a signal even though they are playing????

Thanks