1. One of the most common reasons why a website which has an SSL Certificate installed continues to show as insecure, is that your website continues to pull content, images or videos from unsecured HTTP links. You need to change all the links that you are pulling content from to HTTPS links, and your website will start showing as secure immediately.
  2. The second most common reason why a website may show insecure although you installed an SSL Certificate on it is that your server is outdated and/or doesn’t support the latest TLS settings requirements.
  3. The third most common reason why a website may show as insecure although you installed an SSL Certificate on it, is that you and other visitors continue to open your website through an unsecured HTTP link. You should put a redirect in the server configuration file or in the site’s htaccess file, so that whoever enters your website by typing “www.mywebiste.com” should be automatically redirected to https://www.mywebsite.com. With other words, you should put a redirect that sends all users to your secured site. Here are some articles on how to do this.
  4. You also might be missing the CA-bundle/Intermediate/Root SSL Certificates.
  5. Another problem might be the incorrect SSL installation

All 5 reasons and any other can be revealed by checking how well was your SSL installed using these tools: SSL Server Test and Why No Padlock? 

They will offer you a free report on your SSL Certificate installation along with detailed information on how to fix any vulnerabilities.

Also, we recommend you to read our article called: How to move your website from HTTP to HTTPS easily and with no pain. The article goes even further and comes with many more recommendations on what to check and do to have your website open from an HTTPS link correctly.