Why does a browser say Not Secure after an SSL certificate is installed?

The page may still be loading over http:// even though a certificate exists for HTTPS. Open the full address and verify that the server redirects HTTP requests to the secure version. The application settings, internal links, canonical tags, and sitemap should also use the HTTPS address consistently.

If the address bar shows HTTPS, inspect the certificate for the exact hostname. A certificate for example.com does not automatically cover www.example.com or another subdomain. Expiration, a missing intermediate certificate, an untrusted issuer, or a server presenting the wrong certificate can all trigger a warning.

The browser may also flag mixed content when the HTML page is secure but images, scripts, styles, fonts, or frames load over HTTP. Use browser developer tools to find each blocked or insecure request. Update stored URLs and template code, then clear all caches so old HTML is not served again.

Test in a private window and with an external certificate checker after each change. HSTS and cached redirects can make one browser behave differently from another. If a CDN or load balancer sits in front of the host, verify the certificate and HTTPS settings at every public-facing layer.