Make sure the plug is fully connected to the jack on the mobile phone. Otherwise, some abnormalities such as sound output coming from only one side; or in-line control (if any) being not able to function, etc. would occur.

Note:
The opening for the connection jack in some protective cases in the market is too small. In this case, you may need to remove the protective case before connecting the headphone / headset to the mobile phone.
When connecting a headphone or headset to a mobile phone, attention should be pay to make sure the plug is fully inserted to the jack on the mobile phone.