There are a handful of reasons why the site may show not secure.

If you see an expired SSL certificate warning
If Blackbaud Supports your WordPress site, click Chat with Support and reference this article. Please be prepared to share the following information with Support:
  • URL of your WordPress site
  • The steps leading up to the error message you see
  • The exact error message
If you see a mixed content warning
*Note for more information on how to determine if the SSL behavior is related to mixed content, see Supplemental Information regarding reviewing a site's SSL connection below.
You need to determine the content which is http and updated it to be https. The steps to do this vary from site to site, as each site's content is unique. 

To check for mixed content warnings
Note: this will show you the content which is currently http, which should be updated to https
  1. Open your WordPress
  2. Press F12 on your keyboard, to open a console
  3. Click the Console tab
  4. Note the messages regarding Mixed Content Warnings
Often, the culprit is Image URLs. The images URL may have been manually added to pages or may be within Widgets. You will need to review each of your Image URLs to ensure it is listed as https. 

If your organization uses Luminate survey on WordPress site, see this documentation

Supplemental Information regarding reviewing a site's SSL connection
Here is a 3rd party link by Google on how to check if a site's connection is secure. Note: We provide links to third-party websites in an effort to help you resolve your issue. We are not responsible for the information on third-party websites and we cannot assist with implementing resolutions from these websites.