Mixed content warnings occur when assets still load over HTTP.
This usually happens after enabling HTTPS without updating stored URLs.

Run a search-and-replace for http:// to https:// in the database. Then inspect theme and plugin files for hardcoded URLs.

Browser dev tools help identify remaining offenders quickly. A common mistake is relying only on redirects instead of fixing the root URLs.
The takeaway is that HTTPS requires clean asset references, not just SSL certificates.