I just registered on the site again and didn't get a post to my wall.
Can you let me know each of the settings for Message, URL, and Picture URL that you have for registering and returning user posts? Also, if you can turn on the "Display API Errors On Front-End:" setting in the Facebook API tab, that may show an error, if there is one, when the post is trying to be made.
Usually when the post won't go through, it's because Facebook is rejecting it. That usually happens if the image doesn't exist (you can try without an image) or the URL is invalid or has been marked as a spam page by Facebook (unlikely). If you try with just a message and no URL or Image URL, that will help eliminate those causes for the problem.
Thanks,
Alex