How to get mercurial to notice files added into subdirectories? (hg st, hg add)

If I add a new file into my project's root, it appears with a ? status in hg st, and gets added with hg add.

However, if I add a new file into a subdirectory, it doesn't appear in hg st at all, and only gets added if I explicitly add the file (not even if I add the file's containing directory).

How can I get mercurial to notice files in subdirectories, in a similar way to how subversion notices them?

Thanks