Why does the left side of the headphones work inconsistently? ANNOYING

Sounds like the cable and/or headphone plug has a broken connection. Not much you can do other than replace the cable if you can find a new one.

In my case, I realised if I twisted my right earcup, the audio on the left will start appearing again. I ended up How do I open it up? and realised that if I pushed the white wire inside the headphones into the headband bit, the left side audio starts to work consistently again. So I super-glued that in place, put it back together and all seemed to work. I suspect the portion of the wire that bends when you fold the headphones had failed, and doing what I did forced the conductive material together and also forced another part of the wire to do the bending instead. Not a great solution, but it seems to do the job.

This is actually my second pair of the same headphone, my first pair broke 9 months in so I claimed the warranty. The current pair started to fail in a similar amount of time.