I am an admin of a WordPress network.
One of my blog authors noticed that if she posts a new post, say at /blogname/2011/10/23/my-new-post/ (note--two days ago), that is a member of category x, it doesn't show up when viewing /blogname/category/x/.
What's weird is /blogname/category/x/ shows all the older posts but not the one at /blogname/2011/10/23/my-new-post/.
I've checked everything I know to check. The post's Status is published, its Visibility is Visible, and its Published on is a valid date and time before now.
The blog does use a child category of twentyten. The child theme's categories.php only has trivial modifications--limited to a small style change--but removing it has no effect.
The only thing "different" about this blog is it consists of a few hundred blog posts imported from Movable Type, using the Movable Type and Typepad Importer plugin. I disabled the plugin already.
I'm scratching my head on this one!