You know what!! It was exactly that. The URL was obviously the factor. I changed the permalink to use post title instead of the ID# and it now works.
Another difference is that on your post there seems to be only a Facebook “Like” button instead of a “Share” button like the one on the bottom of my posts.
As for the featured image size.
http://codex.wordpress.org/Function_Reference/the_post_thumbnail#Thumbnail_Sizes