function triggered by “manage_users_custom_column” filter not working

I have the same problem on WP 3.2.1.

There was a filter conflict because a plugin (User Access Manager) calls manage_users_custom_column filter too.

I change priority and it solved the problem :

add_filter( 'manage_users_custom_column', 'custom_user_column_value', 15, 3 );

Hope that helps 🙂