To ensure your RapidSSL certificate is recognized by all browsers, make sure you install the Intermediate Certificates that were provided with the Server Certificate.
Intermediate Certificates are simply “chain certificates” that help devices identify where your certificate came from.
If you need another copy of these, you can freely download them directly from RapidSSL: https://knowledge.rapidssl.com/support/ssl-certificate-support/index?page=content&id=AR1548&actp=LIST&viewlocale=en_US

(In most cases you will want to use the second section, titled RSA SHA-2 (under SHA-1 Root) SSL Certificates, because the new industry standard is SHA-2. If your certificate is still SHA-1 you will use the RSA SHA-1 SSL Certificates section, but we also recommending that you update to a SHA-2 certificate.)

RapidSSL is recognized automatically in many browsers, but some older systems and mobile platforms (Windows XP, iPhone iOS) use these Intermediate Certificates because they ensure recent technological changes with regard to SSL will not have any impact on your site’s security.

We always recommend installing the Intermediate Certificates to ensure maximum compatibility.