It seems that not all Bluetooth devices are compatible with each other. Some sending devices won't let you adjust the volume, while some receiving devices won't let you do it either. So you have to choose your devices so that whichever device, either sending or receiving, you want to have doing the volume adjustment, can indeed do it. When your receiving device won't let you adjust the volume, there's no point in complaining that the sending device won't let you do it either. Choose compatible devices.
The B2 sends the Bluetooth signal at a fixed volume. We've seen people complain that the level is too high, and others complain that it's too low. It seems to me that the receiving device, the one that actually makes the sound that is driven toward your ears, should have the capability to adjust its volume to your liking. If it does not, then you've chosen the wrong device.