Certificate Authorities will almost always require the installation of Intermediate CA certificates to have SSL/TLS Certificates to be fully trusted in all browsers. Think of them as supplementary certificates that help the trust of your SSL Certificate to all the different browsers, and operating systems that exist out in the world.
If you are having Trust issues on certain browsers, and other browsers are fine, then more than likely you’re missing the installation of an Intermediate CA.
Symantec Intermediate CA’s
GeoTrust Intermediate CA’s
Thawte Intermediate CA’s
RapidSSL Intermediate CA’s
If you also require the Root CA certificate to go along with your Intermediate CA you will find links to their corresponding Brands & Products in the above links.
Installation of these Intermediate CA will vary depending on the system. Visit our SSL Installation pages should you need some assistance. If you do not see our server listed then contact your server vender for best practices on installation of this certificate.