Why my email signature is not populating automatically when I select an email template?

If you just start an email the signature will get populated automatically, however if you select an email template the signature will only get populated if it is included in the template itself. In order to add the signature to each template you'd need to edit the email template and insert [userSignature] where you want the signature to appear.

The reason for this is because you may not always want a template to include the user signature.