Navigate to and open the Galaxy Wearable app, and then tap Touchpad. ... Tap Left or Right to choose your desired earbud.Then, choose your preferred control, such as Ambient sound, Volume up, Volume down, Spotify, or so on.
you can't have one bud changing volume. it's either volume on both sides or 2 other functions i.e ambient mode and assistant.
the way I got round this, is to activate the double tap edge feature in Labs on the wearable app so I have volume up and down by tapping the edge, the ambience mode on the left, assistant on the right! on top of the play/pause single tap, skip double tap!!
when this is activated, I just tap the back of my ear for volume.