Because of the nature of the way sigs are displayed on these forums (on their own page, shown in an iframe in each post), your browser may have cached the older version of your sig (assuming you just started using this sig today). If you hit Ctrl+F5, it forces your browser to get fresh versions of the pages you're viewing -- including your sig page. If you view older posts you've made that you viewed before you made changes to your sig and they don't show the updated version, Ctrl+F5 should do the trick for that page. You may have to do this for other pages, if your browser doesn't take the hint and change all cached versions of your sig.
Another possibility is that you used the "Quick Reply' function at the bottom of the thread page for that post. When using "Quick Reply," your sig is not appended to the post by default; you can edit the post and tick the box next to "Append Signature" for it to show up.
HTH!
EDIT: PS - This probably belongs in the Signatures forum. 