I figured out what the cause of this is. In order to prevent duplicates the feed manager is creating a hash based on the title and link of the feed item. Since the forum feed has replies to posts many of them have the same title and url and thus are seen as duplicates and it just updates the existing entry with the same hash rather than adding new entries.
So, the solution will be to append #post + postid to the end of each item url in the forum rss feed so that each post generates a different hash and are not seen as duplicates or updates of the same item.
Best,
Joe