Validation Callback Not Being Called

[eluser]Dirigible[/eluser]
Hi all,
My callback function from validation rules isn't being called
The other validation rules for that field ARE being called -- (required|max_length[12]|alpha_dash)
but not my callback __check_login_name

Full code: $rules['login_name'] = "required|max_length[12]|alpha_dash|callback__check_login_name";

Thanks!