It may happen. If you checked 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 its not clogged
It may very well be just plain simple failure of hardware.
I had before earbuds in which one side started to sound lower, after i checked all these things it was really just failure of hardware, I fought it by 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. It may be same case.
If you check all above and earbuds are still under warranty, return it to store.
If no maybe start looking for replacement (meanwhile you can adjust it as I did by switching balance toward failing side).