Drupal is not designed to aggregate it's own internal RSS feeds, since a direct node query makes more sense, generally.
However, if you want to have internal feeds interleave with external feeds (as we recently decided for the ATS site), Drupal feed urls typically end in /feed/ with no filename or extension.
Go to aggregator feed list. In the URL for the internal feed you are adding, append rss.xml to the feed url. Update item. All is right with the world. This may also solve other problems you may have with other external aggregators.