You have to actually post on the mural for it to show up. If there's no posts on it, then it won't show.
To post to the mural and get the spot started out with a wall, go to the spot under the banner and ícone where it has the various links (Updates, Home, etc.), and click the one that says "Wall". This will take you to the mural and allow you to post on it.
No spots just don't HAVE walls, but some have no mural posts.
