First of all just make sure you’re using Full SSL in your SSL settings. Unfortunately with mixed content, sometimes even that isn’t enough.
You may need to try enabling Auto HTTPS Rewites: https://support.cloudflare.com/hc/en-us/articles/227227647-Understanding-Automatic-HTTPS-Rewrites
Outside of that, follow this guide: https://support.cloudflare.com/hc/en-us/articles/200170476-How-do-I-fix-the-SSL-Mixed-Content-Error-Message-