If you still get mixed content warnings, you have to track down the cause.
If the mixed content fixer is working, the Really Simple SSL fixes all images, javascript, style links, @import URLs and iFrames with http links. In most cases, the mixed content is caused by http links in javascript (.js) or stylesheets (.css) that were included. The mixed content fixer cannot fix these.
Read this step-by-step article on how to track down mixed content. After you’ve tracked down the .css file or .js file, you can change its source from http:// to //, which makes it fit for both protocols.
If you have URLs in your web source causing mixed content warnings that were not fixed while the mixed content fixer was active, please let me know which HTML code caused this, so I can add it to the mixed content fixer.