Ultimately, there is little or nothing you can do: the problem is, an employer is under no obligation to promote any employee, ever (unless, that is, the employee has an employment contract calling for his/her promotion at certain times or after certain length(s) of employment). This is one consequence of all employment being "employment at will" when there is no contract. Therefore, since the employer is not required to promote you, you can't force them to promote you or get any compensation for not being promoted.