If the complaint is for discrimination or harassment based on employee race, color, national origin, ethnicity, religion, age 40 or over, disability, or sex, then if the employer is ignoring those complaints, that would be illegal discrimination and you could file a complaint with the EEOC. But if the complaints are not about harassment or mistreatment based on one of the above factors, the employer may ignore them and you have no recourse: except for prohibiting those few specific kinds of discrimination, the law does not require supervisors to be fair, courteous, professional, etc. Rather, a supervisor can mistreat employees as much as he or she wants (that is part of "employment at will": you have no right to a job or fair treatment at work) and the employees have no recourse but to seek a different job.