There is a difference in volume between the left and right sides, or the bass (low frequency) sound is insufficient.

If you feel that the bass (low frequency) sound is low or there is a difference in volume levels between the left and right sides of the headphones, change the earbuds to another size that comfortably and snugly fit both ears. 

NOTE: The size M earbuds come attached to both sides of the headphones at the time of purchase.