I'm afraid I can't give you an encouraging answer here. The feed appears to be fine, and as you say can be subscribed to manually in iTunes (and from the button on the Store page). The Store simply hasn't updated since after episode 2 and I can see no reason whatever for this.
This sort of thing isn't entirely unknown - the Store has been known to get stuck in individual cases sometimes, and usually in the end it sorts itself out, though it's completely frustrating.
I have seen cases where inexplicably a podcaster's server doesn't react well to requests from the Store, though it's OK in the iTunes application, though usually this happens from the start rather than part way through the podcast's history. If the people running your server are focussed and have proper logs they might be able to find a request for your feed from the Store and see whether an error has cropped up, but it's a pretty long shot.
You could try emailing iTunes Support at podcasts 'at' apple.com, but don't be surprised if you get back a form answer telling you how to add episodes (which you already know perfectly well).
