If its not a source issue and always the same side. You should check the nozzles and make sure oneside isn't holding an ooze of earwax.

It's a little hard to describe because I don't produce that much and when I do earwax in my nozzles its more of like a rocky crystal rather than a soft goop. But I know people can have that issue with buds and IEM's. Could be one ear is over producing it and is muffling the audio on one side.