If the left volume wheel is not adjusting the volume, the headset may be in the wrong volume control mode.
The LS50 headset has two modes for volume control:
Gaming Control Mode
Bluetooth Control Mode
In Gaming Control Mode, the left volume wheel controls the level of the incoming audio from the dongle (connected to your gaming console). In Bluetooth Control Mode it controls the volume level of the audio coming from your Bluetooth device.
To switch between the two control modes, press and hold the left earcup button for 1 second.