Whoops.

As mentioned earlier, I use Varnish to cache static pages on this site to improve performance. I use a WordPress plugin that detects when parts of the site are added or modified (e.g. a new post is published, or an existing one is edited) and it purges the cache for that particular page so the cache content will be refreshed with the new content.
Unfortunately, it was not purging the RSS/Atom feed, so subscribers weren’t getting any updates for several days. That’s very odd.
Until things get resolved with the plugin, I’m manually purging the cache for the feed so subscribers will get more timely updates.
Sorry for the trouble.
Update: What luck! A new version of the cache-purging plugin was just released today and fixes the problem. This pleases me.