Contacts not automatically populating in email compositions with Sequoia

Once upon a time, before purchasing a MacBook Air and selling my MacBook Pro, when I readied an email to send, I only had to put in a few letters of the name, and the email address would automatically appear. That now only happens sporadically, with only a very few email addresses. How do I restore the ability to access ALL emails in my contacts??

[Re-Titled By Moderator]