I'm going to take a guess that you didn't purchase a Wildcarded SSL cert.
specific crap from Geotrust... but any other reseller will likely apply...:
Secure Multiple Sites with One Wildcard SSL Certificate
If you're looking to secure multiple websites with one server certificate, True BusinessID® Wildcard is the easy, affordable solution. True BusinessID® Wildcard can secure multiple fully qualified domains that share the same base domain name if they share one physical server.
For example, if you need to secure the following websites:
and all of these websites are hosted on the same server box, you can purchase and install one Wildcard certificate issued to *.yourdomain.com to secure all these sites. The number of sub domains that you can use the wildcard certificate to secure is unlimited, for example, the same certificate can be used every time you add a new sub domain to yourdomain.com. If you need to span the wildcard certificate across multiple physical servers, you may purchase additional licenses.
If you want it specifically for the subdomain https://secure.website.com then you must order it using secure.website.com not website.com.
It will only work for the domain you order it for.
If you want it to work on a subdomain then you have to setup the subdomain up on a dedicated IP address then generate the csr using the subdomain name. order the certificate using the subdomain name and then install it.
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