There's a checkbox to append a signature to a given post; it was turned off for this post -- you'll see a signature appears in it because I edited it and checked the box. There's a preference that applies to the setting of this box for new posts, but it seemed to have been checked already -- maybe you already found it? In any case, I poked it to make sure. Let us know if you continue to have a problem.
In signatures, [i] and [url] can be used; the others won't work in order to keep the signatures from becoming too distracting. That's also the reason why they can no longer contain raw HTML, and why we're asking members to keep them short (no more than 2 lines).