It's not uncommon to encounter a "Not Secure" warning in the address bar even after a website's SSL certificate has been successfully installed. This "Not Secure" warning is a comprehensive assessment of the page's security status by the browser, with the SSL certificate being just one component. A multi-faceted, systematic investigation is needed to pinpoint the specific cause.

First, the most common and prevalent reason is known as "Mixed Content." This occurs when your website's main page loads via secure HTTPS, but some embedded resources (such as images, JavaScript files, CSS stylesheets, audio/video files, etc.) are still accessed via insecure HTTP. This is like locking the main door (HTTPS) with a very strong lock, but leaving the windows (HTTP resources) wide open. To ensure user security, the browser will classify the entire page as "Not Secure" because it contains insecure elements. Diagnosing this problem is quite simple. On the page displaying the "Not Secure" warning, press F12 to open the Developer Tools, and then switch to the "Console" or "Security" tab. This section usually clearly lists which resources are loaded via HTTP, causing the mixed content issue. The solution is to access your website's backend, database, or source code and replace all resource references with HTTPS, or use relative paths, which automatically inherit the protocol from the main page.

Secondly, we need to carefully check the status of the SSL certificate itself. A valid certificate doesn't just mean it's installed. Click the "Not Secure" warning in your browser's address bar, then select "Certificate is valid" to carefully review the certificate's details. You need to confirm the following key points: First, has the certificate expired? Certificates have a defined expiration date and will become invalid after that. Second, does the domain name bound to the certificate exactly match your website's complete domain name? If you applied for a certificate for `www.example.com`, but users access `example.com` (or the version without www), then the certificate is invalid. Third, is the Certificate Authority (CA) that issued the certificate trusted by the operating system and browser? Some self-signed or free test certificates may not be widely trusted. Resolving certificate issues typically requires contacting your certificate provider or hosting provider to renew or reissue the certificate, and ensuring that the complete certificate chain, including intermediate certificates, is correctly installed on the server.

After ruling out content and certificate issues, the next focus should be on server configuration. A poorly configured web server, even with a valid certificate, cannot properly establish HTTPS connections. You need to ensure that the server is listening on port 443 (the standard port for HTTPS) and that virtual hosts are correctly configured to use your installed certificate files. A crucial configuration is to force all HTTP traffic to be redirected to HTTPS. This can be achieved by adding a simple rewrite rule to the server's configuration file. For example, in the widely used Nginx server, you can add the following configuration:

server {
listen 80;
server_name yourdomain.com;
return 301 https://$server_name$request_uri;
}

This configuration means that when someone accesses your website via HTTP, the server will automatically return a 301 permanent redirect, directing them to the corresponding HTTPS address. For Apache servers, you can add a similar rule to your website's `.htaccess` file. Furthermore, modern security practices recommend enabling HSTS (HTTP Strict Transport Security). This uses a special response header to tell the browser that communication with the website can only be done using HTTPS for a specific period in the future, effectively preventing SSL stripping attacks.

Finally, some easily overlooked details can also cause problems. If you are using a Content Delivery Network (CDN) or reverse proxy service, you must ensure that your SSL certificate is correctly uploaded and enabled on the CDN's server. Otherwise, the connection between the user and the CDN may be secure, but the connection between the CDN and your origin server may be insecure, which will also trigger warnings. Similarly, if your website embeds an iframe from another third-party domain that itself uses the HTTP protocol, this will also pollute the security state of your page.