Update: It turns out this problem was caused by plugins. The Wire plugin is causing it on both sites, and the bookmarks plugin is causing it on one site. If you have this problem, I recommend turning off your plugins and enabling them one by one to see which one is causing the problem.
I complain about Elgg a lot, as you can see here. I want to like it, but it does have a lot of problems. I know it is Open Source, but it is also being actively worked on every day. So in my opinion, if there is a known bug in a prominent feature of the site, it should be fixed.
There is currently a major bug in the Latest Activity feature of the Elgg software. I have 2 sites running Elgg, and neither of them display the latest activity on the latest activity page or on the home page. Latest activity is blank on both sites. They were both upgraded and not fresh installs, which seems to be a common factor. Also, I was running the custom index plugin on both sites, so I didn’t notice the problem until I disabled the plugin.
This is a known bug that has not been fixed yet. As far as I can tell, it was reported at least a year ago here. There is a thread on Google Groups here. Here is the bug ticket for this in Elgg’s bug tracking system that was reported 3 months ago.
How does Elgg handle bug fixes? There is a major bug in Elgg’s latest activity display, which is a major feature of the site. They know about it, and it has not been fixed yet despite 2 discussion threads and a support ticket. Hopefully, they will fix it in the next release whenever that is.