Just wanted to share, as i've been wondering it too:
There's a place in the discussion where you can manually subscribe, as it doesn't do it automatically and you don't receive notifications untill you think to check it by yourself.
You can click on the "discussion tools" and subscribe to the discussion. It will also offer you to be notified by instant emails, or by the control panel when you log on HV20.com. See attached.
Hope it helps guys, i've seen a couple of user wondering how and i've recently figured out myself, just wanted to share.