Adding class to menu items doesnt' work

Hi,
the aim is to add classes to menu items.
Accordingly to the codex I added this function to my function.php:

function special_nav_class($classes, $item){
     $classes[] = "new-class";
     return $classes;
}
add_filter('nav_menu_css_class' , 'special_nav_class' , 10 , 2);

but nothing happened.
I mean I checked firebug and no “new-class” is added to the menu items.

Why? How can I make it working?

Thank you