It is possible that your headphones have not been paired correctly and that is why only one is emitting sound. To fix this, follow these steps:
Important: make sure that the coloured strips that cover the charging contacts of each ear piece are removed and that the batteries have been charging for a while.
1. On your mobile: remove previous links Settings-> Bluetooth -> (delete, forget ...)
2.Put the headphone base to charge.
3. Put the earphones inside the base with the cover closed and leave them charging for more than 2 minutes.
4. Hold down the button on the bottom of the base for 7 seconds.
5. On your mobile: turn off the Bluetooth.
6. Remove both earphones from the case to automatically pair them with each other.
7. They should start blinking for a few seconds and then one will stop blinking, that means they have been paired.
They can now be linked to the phone.
7. On your mobile: turn on the Bluetooth.
8. Go to Settings-> Bluetooth, search for the earphones to connect.