Just a thought: is it possible that you have a number of line returns above your signature and that the text is simply being pushed up and out of sight?

I don't have any line breaks or line returns above my signature, and the problem exists regardless of which of my signatures I select. If I select the signature first, before writing the message, I suppose that would work, but I'm not accustomed to doing it that way and I'm an old dog and you know how the saying goes about new tricks.