There are a few reasons that you might not be seeing a post in the hashtag feed that has the hashtag.
1) The posts are from private accounts. In the past, hashtag feeds would sometimes show posts from private accounts. It looks like Instagram changed this recently so that only posts from public accounts show up
2) On Instagram.com the posts are sorted by "top" posts and then the most recent posts are shown. Instagram's API that we use to create feeds with in our plugin will only send photos in chronological order of when they were last edited.
