When you migrate from HTTP to HTTPs, there are chances that a few pages, especially old ones remain on HTTP URLs.
You can fix these problems using the Really Simple SSL plugin or the SSL Insecure Content Fixer plugin. These plugins will take you through the necessary steps to resolve any issues. Then the warning ‘This site is not secure’ will be removed from your site. You’ll see the padlock and ‘https://’ in your address bar.