There are a few possible reasons for this:
1) The certificate has a Common Name (CN) of 'domain.com' but you are going to 'www.domain.com'. In order to correct this, you must have a redirect (consult your web server software manual on how to setup the redirect) from the non SSL portion of your site ('www.domain.com') to the SSL portion of your site ('domain.com').
2) In Apache, it is possible that the 'server name\server alias' setting does not match the Common Name (CN) in the certificate. This can be corrected by updating this setting in the Apache server to match the CN.