Try the following:
- Lift your finger, and then tap the tap sensor rather firmly.
- If you do not put the headset's units on both of your ears, you cannot adjust the volume, even if you tap the tap sensors.
- If the tap sensors are disabled, you cannot adjust the volume. Use the following procedure to reactivate the tap sensors.
- Put the headset's units on both of your ears.
- If the Bluetooth function is on, hold down the button on the left (L) unit for 2 seconds to turn off the Bluetooth function.
- Hold down the button on the right (R) unit for 4 seconds.
- You can also reactivate the tap sensors when the headset's units are set in the charging case.