add_rewrite_rule Ignoring Query Variables

Hi,

I have followed about every bit of advice I can get my hands on but I cannot get add_rewrite_rule to process query variables.

I have a query variable ‘n’ which I have registered with wordpress. I am using a very simple rewrite just to test it is working.

If I go to this url manually it works fine:
index.php?pagename=materials&n=Clay

But going to:
resources/materials/Cloth/

Just takes me to:
index.php?pagename=materials

Ignoring the query variable.

This is the code I have (to test with):

//Custom Query Vars
add_filter( 'query_vars', 'add_custom_query_var' );
function add_custom_query_var( $vars ){
  $vars[] = "n";
  return $vars;
}

add_action( 'init', 'wpa5413_init' );
function wpa5413_init()
{
    add_rewrite_rule(
        // The regex to match the incoming URL
        'resources/materials/Cloth/?',
        'index.php?pagename=materials&n=Clay',
        // This is a rather specific URL, so we add it to the top of the list
        'top' );

    add_rewrite_tag('%n%', '([^&]+)', 'n=');
}

//Flush rewrite rules before adding new ones.
add_filter('init','flushRules');
//
function flushRules(){
    global $wp_rewrite;
    $wp_rewrite->flush_rules();
}