Start by trying to determine what is causing the mixed content flags: It may be scripts or advertising (including AdSense) as well as images. Test your site here:
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Address any errors you see there. Sometimes, just replacing http:// with simply // will do the trick in scripts and plugins.
Also, even if it's only images, you won't see an immediate result. You'll need to allow some time to scan all your images - and that may take a while if you have a lot of them.
Are you seeing any entries in the cache-img folder?