I have a user with a strange situation. In some posts we see the signature but in another posts (before to configure signature), no.
Why? I supose that updating a signature takes effects on all user posts!
Did the user answer via quick reply in one variant and full reply the other?
Have you edited the reply templates in any way?
I can't contact the user right now but it may have used both reply ways.
I don't remember exactly a code edit, but in April 2012 I searched for a solution to appear the format bar on full reply.
I can make something now in order to solve this difference?
Check the new reply, new thread, edit post/thread and quick reply.
If they all have the option to display signature then they are fine.