in your Admin,go into users and groups,search users,select modify on your name.In permissions go down to ' always attach my signature' and select yes.Save and go back to your forum .It will only show your sig in new posts not old ones.Hope that helps.
did you set it up in your profile for sig to work? know dumb answer. Also might try one more time to re save in admin panel. Finding have to save many times in order for it to work.
good luck it happened to our forum and took a week for it to get back to normal.