Mail signatures

I have to have my signature at the bottom of my emails, and have created the signature which is appropriate for my email address in Mail preferences. However, when I compose an email the signature is not there - I have to choose the signature from the drop down menu, which is a pain as I send heaps of emails daily! Any clues as to how I can fix this please? I'm using Mohave on a desktop iMac, if that makes a difference?