Many times, though you have set the site as secured, your user may see the warning of site being insecure when they visit it. This can happen when you site is secure, however, your products, or pages or categories are using non secure elements. Like a non-secure (http) link for an image that has been uploaded or if you are using a CSS to change the styling, it might be using a non-secured link.
In order to find those links, the easy and quicker way is to use Inspect command in the browser (right click on the page and select inspect), opening the console and reloading the page. This will generally show the not secure elements.
Another option is you can look for Page Source of your site. If you open on a browser, right click and select the option of Page Source:-
Once you click view page source, you will see your page in the form of a code. All you need to do is search for any links that shows http:// and not HTTPS://. The non-Secured links may have been added to long description or Category or Products or Pages or it may have been added to the HTML head content of your site settings. Once you change these links from HTTP to HTTPS the site will again start showing as a secured site.