I'm not sure about your particular BT device. I use the BT audio on my car. What I found is that the volume control on my car stereo acts independently to the volume control on my phone. I have to set the BT volume on my phone to max and then adjust the volume accordingly on my car's stereo.
If your BT headset has its own volume control that is independent of the phone, wait for your BT to connect and are on a call. Then adjust the volume level on your phone. As you adjust the volume level on your phone, you will notice the volume indicator for BlueTooth will appear. Max that out. Then see if you can also adjust the volume level on your headset and see if it is now at a loud enough volume.