Activity stream doesn't show forum posts

Hi. I have two groups, set up identically, and both have forums.

Group A displays its forum content in the activity stream

Group B doesn't display its forum content in the activity stream 

From what I can tell, both groups have the same set up. Both groups have activity streams with the same options and same filter settings. The forums in each group have the same settings.

The only difference I can see is that Group A has forum posts as recent as the past couple of weeks, and Group B hasn't had a post since December. Nevertheless, Group B's activity stream still shows other content posted in the the group going back months. Just not forum posts.

Any ideas?