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Site Shows "Not Secure" Despite an SSL Certificate: 7 Causes and Fixes

The certificate is installed, yet the browser says "Not secure" or shows a warning page. The seven most common causes, from mixed content to a missing redirect, and how to fix each.

3 min read

When a site appears as "Not secure" although an SSL certificate is installed, the reason is almost always one of seven. The most common are mixed content, a missing redirect from HTTP to HTTPS and a certificate that does not match the name being opened.

First: which message do you see?

What the browser showsLikely cause
"Not secure" in the address bar, page loads normallyPage loaded over HTTP or contains mixed content
Full-page "Your connection is not private" warningCertificate expired, wrong name or incomplete chain
Warning on some devices onlyIntermediate certificate missing or wrong device clock

1. Mixed content

The page arrives over HTTPS but loads images, scripts or fonts over http://. Fix: open the browser’s developer tools (F12), look for "Mixed Content" messages in the console and switch every included address to https://.

2. No redirect from HTTP to HTTPS

The certificate works, but visitors land on the HTTP version through old links. Fix: set up a permanent (301) redirect on the server, then add the HSTS header.

3. The certificate does not match the name

The certificate covers www.example.com but example.com or a subdomain is opened. Fix: use the SSL checker to see which names are in the certificate and reissue it with all the names you need. For many subdomains a wildcard certificate fits.

4. The certificate has expired

Certificates are valid for at most 200 days; after expiry the error NET::ERR_CERT_DATE_INVALID appears. Fix: reissue and install; from now on plan 30 days ahead or automate.

5. The intermediate certificate is missing

The server sends only its own certificate, not the chain. Desktop browsers often compensate; mobile devices and apps do not. Fix: install the certificate together with its intermediates in the right order.

6. The wrong certificate is served

The new certificate is on the server, but a load balancer, CDN or second virtual host still serves the old one. Fix: update the certificate everywhere TLS terminates and reload the service.

7. A problem on the visitor’s device

If only one device shows the warning, its system clock is often wrong or security software is intercepting the connection. Fix: correct the date and time.

Checklist

  1. Does the page load over https://? If not, set up the redirect.
  2. Check expiry date, names and chain with the SSL checker.
  3. Look for mixed content in the console.
  4. Cross-check on a second device and on mobile data.

The certificate decoder shows the names, issuer and validity of a certificate you hold.

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