Did you use to have TWTCommish's hack? There may be conflict as they use the same table names. Your problem is strange as they can see the shoubox.php page, so it's not a permission problem. But when they post a shout, nothing shows up. Did you change anything at all to the hack? Did you run the queries to install the tables?
One thing I always try when I get a bug that I don't know the cause of, is to delete everything (all files, templates and db entries) and start over. It usually works.