I have set up a signature in my account profile, but it doesn't seem to work.

... and, of course, it worked on the above posting about it not seeming to work !

... but not on the above response.

And not on this one.

Signatures are inconsistent. This is an old issue. From the known issues list at the top of this forum:

3. Signatures don't behave properly: We know, it's an old issue. No ETA on when it's to be fixed, but we have documented it exhaustively, so there's no need to report it. Signatures show sometimes, and sometimes not.