When you post, if you look down where it says Automatically parse URLs, Email Notification, etc. You will see "Show Signature." See if that is checked or not. I hope I don't sound dumb, it's just that that's what happened with my friend on PT. Her sig wouldn't appear in her posts because 'Show Signature' wasn't checked.

Anyway, if you can't get it, you can go to this thread and it may help you.