buddypress activity stream - some new posts don't appear anywhere on site

I've noticed that some new buddypress posts don't appear anywhere on my site (or just on a specific activity post page).

I receive notification emails that users have commented on an activity post or blog - then click on the link in the email and nothing is there, nothing is on the user's activity wall and nothing in the site's main activity wall.

Over the past few weeks I've had to 'switch to' that user and use the comment or update that you can see in the email notification to post as that user. It's not very time effective and I'm afraid we're missing questions and activity updates on the site.

Has anyone else experienced this or have any ideas on what to do/check?