I have several custom post types set up on my site,

I have several custom post types set up on my site, and I'd like to pull a feed from each of them separately. However, only some of them seem to work using the http://mysite.com/feed/?post_type=myposttype format. For example, I can pull a feed for the authors post type I created http://indiewritenet.com/feed/?post_type=authors, but not a feed for my books post type: http://indiewritenet.com/feed/?post_type=books.

Any suggestions for clearing this up are appreciated.