I followed this tutorial to the tee to obtain a wildcard certificate. After that, the primary domain has the padlock symbol but all my subdomains say "Not Secure". I had certs for the subdomains earlier which expired. I deleted them all before getting the wildcard cert. I'm on an Ubuntu 18.04 server.
You chose the manual method for getting a cert which means you must update your server config by yourself. You have not done that. Or, maybe you did but you need to reload Apache after making such changes. Your server response headers say you are using Apache so you should review your VirtualHosts.
Specifically, your server for the apex domain (rajnarayanan.com) sends a current cert but it only has the www subdomain in the cert. You can see which cert your server sends with a tool like this SSL Decoder
The VirtualHosts for the other two subdomains send an cert that has expired.