The SSL installation provides cover for all the website files and folders that you have on the web hosting server. At times, some websites comprise of external resources like Google Adsense campaign, banners of other websites or links to other websites that are not secure. In such situations Internet Explorer and other browsers display a warning that your website contains both secure and non-secure items.
This issue has nothing to do with the SSL certificate installed on your website. In order to resolve this issue, you will have to inspect your website code and make sure that all the banners and files from external resources are either added through a secure connection or completely removed.
Check out the SSL certificates provided by MilesWeb and choose the one that suits the nature of your website!