In many cases when you install an SSL certificate for your website and reconfigure it to open via HTTPS, there are still items on the website's pages that are requested via HTTP. This can be internal resources situated on your hosting account, or external ones, pulled from remote locations, like Google fonts and similar. Having resources on your site that still load via HTTP prevents the green padlock from showing in the visitors browser.
For WordPress you can use our SG Optimizer's Force HTTPS functionality. It will redirect all the traffic for your website to HTTPS which should help avoid mixed content, except in some cases of remote resources still being pulled over HTTP.
If you are not using WordPress or even after using our plugin there is still mixed content on the website pages, then you can use this online tool to find which content is being served via HTTP. You would have to attempt to correct all of them to load over HTTPS manually, based on the specific elements.