It’s highly unlikely that it’s a bug in the activity stream – hundreds / thousands of sites run the BuddyPress activity stream without problem.
It’s almost certainly something wrong in your bespoke theme / bp-custom.php file.
Have you changed the theme since the second person carried out their activity? Does it show in the main site activity stream? Have you installed some privacy plugins, or has the member specified that their activity is private? Is the site hidden from search engines?
It’s impossible to provide much more help without a lot more information about your site configuration and the details of your theme code.
The theme hasn’t changed in months, but the person’s activity does not seem to appear on the sitewide stream. No privacy plugins – or blockage of engines. Dumb question- how do you make your activity private?
“activity private” – was just a general thought if you’d added a plugin or other custom code.
As I said before, without detailed info about your configuration, and your theme – it’s going to be difficult for anyone to help you on this one.
Does it only affect one user, or more than one?
Can you simulate it on one machine / browser – i.e. try activity for 2 users – do they both work?
Most users don’t seem to have the issue, but having seen it on one, I am concerned that it could be happening to others….but I can’t check the several hundred users individually – although a quick glance of a few dozen shows it’s only on this one person. How strange. What more information can I offer you? We’re running WPMU 2.9.2 and BP 1.2.3 – we’re going through a major upgrade where we will go to 3.0.1 and 1.2.5