New posts not showing up in category views

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 plugins we’re using for this blog are Unfiltered MU and Viper’s Video Quicktags.

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!