This seems to happen on a regular basis. WordPress simply stops listing new blog posts under category headings. No explanation, no nothing.
is not listed at any of the 12 categories I've listed it under. (This is a new secret limit on category listings that is imposed by WordPress administrators without telling anyone about it until they complain, as I found out last time I turned to the forums for help -- at http://en.forums.wordpress.com/topic.php?id=31424&replies=9#post-229117
This has happened before. The only solution anyone mentioned was changing the language in "settings", logging out, then going back in and re-setting the language in English. (This "fix" no longer helps -- didn't help last time, doesn't help this time.)
I also tried re-editing the post, changing the number of categories, re-saving it. Didn't help.
This is actually one of the reasons I started blogging at WordPress, but it seems to stopped working A LOT ON A REGULAR BASIS.