XF 1.4 Guests only seeing partial recent activity stream?

I use the recent activity stream as the default landing page (we're not your usual type of forum).

This works great for registered members, but for guests they only seem to get a partial stream, with the vast majority of posts missing.

I have played about with all the permission settings, even testing giving guests full mod privileges (for a few seconds) but still the recent activity stream remains only partially populated?

Am I missing something obvious?

What's odd is that the content that does appear in the feed spans all our forums, but seems to only include some posts, whilst missing others completely. Logged in users get everything as you'd expect.
Not sure if relevant but previously we've only allowed registered members to see anything at all. Guests could not even view the nodes. Could it be I need to force some sort of refresh?
Is there any pattern to the content they can't see?
Not that I can see. Initially I thought it was posts that had been moved from one forum to another that weren't displaying, but then I realized one was there - just not all.

I just posted a test message in all our forums, and they all displayed for guests in recent activity.

Could it be linked to the fact until just now guests could not see anything? I've only just changed the settings to allow guests permission to view forums. Now they can view everything registered members can.

Not sure why some old posts would display but not others, but is this a case of the database being out of sync? Can you force refresh the latest activity feed?