Need help troubleshooting Solo 3 Beats headphones

When used wirelessly, sound is full and present in both ears.

When cable is plugged into Solo 3 Beats headphones sound only comes into one ear .. if I hold the round plug that goes into the headphones and push it in, sound comes into both ears but I can't keep holding it. Wondering if there is something wrong with the end of the cable or the connected within the headphones. They are only 2 years old and work with a cable until 2 months ago.