Some devices do not support volume synchronisation. That means you can adjust volume independently on both of your connected device as well as on the The Headphone. On these devices we recommend to set the phone volume to maximum so you have the freedom to adjust the full possible volume range directly on your Headphone.
iOS: Also check your EQ and volume limit settings on your phone and make sure that both are turned off since they might affect the overall volume as well. Settings -> Music -> EQ/ Volume Limit
Android: Also check your Media volume settings if volume is set to max, make sure EQ and other apps like Audio Effects are disabled, since they might affect the overall volume as well.
If the volume is still too low, please consider that we are bound to regulations regarding the volume so we have engineered the sound according to European safety standards.