SSL Check
This online tool will scan your website for non-secure content (i.e. non-secure images, scripts and css-files that will trigger a warning message in browsers). The number of pages crawled is limited to 400.
https://www.jitbit.com/sslcheck/
Make a note of any web pages which are flagged as insecure. Then use this tool to identify the problem:
Why No Padlock?