Some previous suggestions:
- Checking your own ears (switching earbuds left<>right).
- Checking if your media player/sound card have sound balanced (same level for left and right ear).
- Checking if the earbud is clogged.
It may very well be just plain and simple failure of hardware. I had earbuds before that had one side starting to sound lower. After I checked all these things, it was really just failure of hardware.
I tried adjusting balance (put it more to side which got lower volume and increase overall volume) But, it kept decreasing until it was so low that having balance completely to that side had no effect. Your issue may be the same case.
If you check all the steps above and the earbuds are still under warranty, return it to the store. If not, maybe start looking for some replacements (meanwhile you can adjust it as I did by switching balance toward the failing side).