nav_menu_css_class filter not working

I’m having trouble getting a filter on nav_menu-css_class to work and I can’t figure out why. I’m using the this example on the reference page for wp nav menu.

<?php
add_filter('nav_menu_css_class' , 'special_nav_class' , 10 , 2);
function special_nav_class($classes, $item){
     if(is_single() && $item->title == "Blog"){ //Notice you can change the conditional from is_single() and $item->title
             $classes[] = "special-class";
     }
     return $classes;
}
?>

For testing purposes, I changed it to this to try to get it to work at all:

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

     return $classes;
}

I would think with out the condition statement, it would add the “special-class” to every nav item, but its not. It doesn’t look like its getting called at all? Can anyone help me figure out why?