Check for insecure content
In most cases, the reason is that your website or the page has 3rd party insecure links and content. That could be images, CSS styles or scripts that are embedded from 3rd party websites without an installed SSL certificate.

Click "F12" in your Chrome browser;
Find "Security" tab;
Check if you have any issues.
The screenshot below shows that connection is secured and the SSL certificate is valid. However, the "resources" shows "mixed content" confirming the insecure content on your website.

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Another screenshot that shows how the insecure content looks in the HTML code. You need to find all such lines in the code of your website and change them from HTTP:// to HTTPS://, that should fix the problem.

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