mugglesI've had this same problem - its due to a broken connection in the left earpiece. If you have access to a soldering iron, it can be fixed in about 15 minutes. Rotate the perforated center cap on the outside of the can to expose the two philips-head screws. Remove the screws and pop off the plastic cover. This exposes the wires below. The wire is a very small gauge and flexes a small amount when handling the headphones. This eventually strains the wire at its connection points and can cause the strands to break. Carefully strip back the insulation on the broken wire and re-solder it to its post.
I had to do this twice - the 2nd time I replaced the broken wire with some thicker wire I had on hand.