In the Web Security Service portal policy editor, a new added URL is used. However, the service blocks it because of a category rating in the G4 layer.
When creating the rule adding the URL as http://www.domain.com might not be necessary or correct; the "www" might not always be appropriate.
A site added to the content filtering rules as www.domain.com might actually land on http://domain.com. However, pre-pending www to the domain might cause the rule not to match. Always confirm the landing page in the URL field of the browser.
Also, it is not always required for you to add http:// or https://, if the desired site is accessed by redirection then these might apply. Start by simply adding the top level domain (TLD) to your rule set and then prepend/append as necessary.