One of my Groups is not notifying some of its members. Is there some way to diagnose?

I have a few discourse Groups set up. Most act normally but one in particular has some behaviors that perplex me. The members of this Group are almost exclusively users that use email. Not technically inclined.

  1. Emails to the Group only get distributed to a subset of the members. It’s always the same members. I have asked members who are not being sent emails to do things like send a test email, or create a PM to the Group using the UI. This does not cause them to start getting emails. Also, when I create a new user and add them to the group, they do get emails. An @ mention to the Group does seem to trigger notifications.
  2. The members that don’t get emails also don’t get notifications of any kind. That is, the green counter doesn’t appear in their avatar icon. They have to go the the Group messages to even find out a new message has been sent. (Not something that these users do).
  3. I don’t know if this is related but when I view the PM inbox for non-notified members of the Group, the group name does not appear in the navigator list (Inbox, Send, Archive, etc.). For other Groups there is a navigation link that when clicked only shows message for that Group.

It only happens for this one Group. I could potentially recreate the Group from scratch but I fear doing so would be a bridge too far for a lot of tenuous users. I’d prefer something that I can do on the Admin side that will make it start working for them again without asking them to follow any potentially bewildering instructions.

The only thing I can think of that might have caused this behavior is that I had at one time played around with having the Group act as a mailing list backed with PMs vs. using a Category with an email address associated that is accessed only by the Group. I think I switched back in forth between those modes before settling on just PMs.

It feels like a bug to me but I don’t know for sure. Is this behavior something that can be explained by the normal function set of Groups? Any suggestions on what I can look at? I have not been able to reproduce a new user that doesn’t get notified but of course I can examine user accounts that are non-notified.