People Picker problem in sharepoint 2010

I've added the membership and role provider keys for an FBA database into the web.config of a standard AD authenticated SharePoint site (as per Andrew Connells article).

This should then allow me to add roles and user to the site that can then be subsequently used to log in via an FBA configured Url.

When I add in a Role or User it should become underlined in the people picker dialog of the add user page, to show that the role/user has been recognised.

However this is the problem, I cannot get the roles or users to resolve and 'underline'. I have other sites configured in exactly the same way and working correctly, I just can't seem to get this site or any newly created sites to work as expected.

I've checked and rechecked my web.config entries, I've even pasted the entire web.config file from a site that works correctly into a problem site but I still can't get the user or role to resolve.