The official RSS feeds given by Google (http://www.google.com/support/blogger/bin/answer.py?hl=en&answer=97933) only retrieve the latest 50 posts and 20 comments, even if the settings under Site Feed has been set to Full. This default behaviour is useful as readers are usually only interested in latest posts, but will be a problem if you somehow want the RSS feed to show your entire blog.
The solution is simple, just add a HTTP GET parameter to the official RSS URL. For example, if your RSS feed is:
The feed that returns the full blog is:
Same for comments, add the max-results parameter to retrieve all comments:
where N is the maximum number of posts/comments you want to retrieve.