Only getting sound from right hand side of sound syatem

Hi Guys, the past two week when I have turned up for my regular saturday evening slot I have had problems with Serato.

Week 1) The 1st week I had issues I came in, set up my SL3, fired up the sound system and had no sound from the left hand side of the rig. The master volume on the mixer was also showing only lights on the right hand side. I checked & swapped out RCA's for new ones and still no difference. I shut everything down and re-booted, unplugged everything and reconnected it all still to no avail. I did this a further 3 or 4 times and eventually it worked!

Week 2) Saturday evening past I had exactly the same issues only both left & right light were lit up on master volume but still no sound from the left hand side of the rig. I tried my usual routine of unplugging and re-booting but it didn't work this time. I had to roll with a half assed sound system. :-(

I'm running a brand new MBP 13" and 2.4.3. I never had any issues like this with my old Acer laptop.

I should add that the sound system works absolutely fine when running for the other DJ's and the Friday guy uses traktor and has had no issues.

Really hoping you guys can help me out.

Thanks in advance.