Your first and third site have a TLS certificate, but don't have a redirect from HTTP to HTTPS:
Your webserver needs to implement this redirect, otherwise the browser doesn't know it can and should load the HTTPS version. You could try
Your second site also has a TLS certificate, but on top of lacking the aformentioned redirect, it includes HTTP content: summary.gif is loaded through HTTP which should be HTTPS. If content is loaded through HTTP, most browsers nowadays mark the site as insecure, even if a valid certificate is present.
Thanks so much for your help Osiris, I really appreciated it.
I have followed your suggestions, didn't really know what I what I was doing, but it worked. All my sites are now showing as Secured regardless of how I enter their names.
I ticked the redirect HTTP to HTTPS box in Hosting Settings in PLESK. I also changed the HTTP content for summary.gif to HTTPS.