Hi Alan,
I’ve narrowed this down to an issue with bbPress and how it interacts with WordPress’ site privacy setting.
You had your site set to disallow search engines from indexing your site. (View the “Reading Settings” page from the WP admin dashboard for this setting.)
bbPress also uses this option to determine if the forum item can be recorded into the BuddyPress activity stream. Since you disallowed search engines from indexing on your site, no forum-related activity was ever recorded. And since the Group Email Subscription plugin relies on forum posts to be recorded into the BuddyPress activity stream, no subscription emails could ever be sent.
In the meantime, if you allow search engines to index your site, you’ll be able to get group emails for forum content working again.
I think this is a bbPress bug; I’ve just filed a ticket about this.