Where does Mixed Content come from?
It usually comes from absolute HTTP links stored in a theme, CMS database, old widgets, advertising scripts or manually inserted images.
Browsers may block some of those resources, causing the page to look different or lose functionality.
How to fix it
Change resource URLs to HTTPS, force HTTP-to-HTTPS redirects and check CMS settings such as WordPress Address and Site Address.
After fixing it, browse several pages and check the browser console.