KP3 strange cable issue

[CountFractal]

This is purely a curiosity issue and I'm not sure if anyone will be able to help, but hey...

Basically, I'm running a mixer into a laptop and then running the laptop output (including the mixer output) into my Kaoss Pad. The strange thing is, one of the cables that I use to my KP3 LINE IN from the laptop gives no left channel signal when heard through my KP3 OUTPUT.
I'm pretty sure the cable is fine because it gives both channels with any other piece of gear I plug it into and - oddly enough - other cables that I use work fine with the Pad.

So, as far as I can see there is no problem with any of the parts but the issue with lack of a left channel signal persists.
This issue, whilst being completely harmless (I just swap the cables) IS driving me mental - I like to know WHY things are happening!
Has anyone else experienced anything like this and could explain it so I can stop constantly churning it over in my head?