Update: After some digging I realized, that the action hook 'publish_post' does not fire, when a custom post type is published. One has to use another hook like 'publish_{{custom_post_type}}' to catch the publishing and kick-on the push to the hub.
In my case I added
add_action( 'publish_news', array( 'PubSubHubbub_Plugin', 'publish_post' ) );
to the init() funtion in pubsubhubbub.php
I noticed that the <atom:link rel="hub" […]> tag is missing in my custom post type feed xml. Maybe that helps narrowing down the problem?
You are welcome!
Im using your amazing plugin on production site already, great work.
Keep on hacking!
