The topic showed in the feed the first time it was created.... It then showed in your reader and you marked it as read.
When a reply is posted, the topic timestamp is updated and it will reflect this change in the feed. But Google Reader recognises this as a previous post, and one you have already marked as read, so it won't show it again.
I am flying out to attend a conference, but will check if there's a way to make RSS entries "unique" and perhaps add that. This should make the reader shows the entry when it's updated, even if it's an old topic already seen.
It's been a while since I've worked with RSS but I think there's a "guid" field that you can use as a unique ID. Of course, whether it's easy to integrate that with the forum software could be another matter entirely! :)