It sounds like you were tugging at those wimpy 3.5mm interconnects and then "pop" goes one channel. You know; just "halfway out" of the jack. Then a little wiggle and "pop" it goes back in. Or maybe it's barely making contact in the first place. Or there's a short in the cable, which isn't at all out of the question. Or one of the jacks has broken loose from its position on the circuit board. Again; your problem is the 3.5mm interconnects.
And, as boojum said, these problems are well-nigh impossible to diagnose without seeing the hardware firsthand. Since you happen to have the hardware handy, it's easiest for you to do your own troubleshooting. And boojum's "wiggle check" is just an easy way of checking for electrical continuity in the cables and jacks. The good news is that repairing cables and jacks is relatively easy.