If you mean that you are being paid less than others due (presumably) to your sex, race, age over 40, national origin, disability, or religion, then contact the Equal Employment Opportunity Commission or EEOC to report illegal workplace discrimination. If you mean that there is no illegal discrimination of the above kinds going on, but that they are violating their internal pay policies, the law does not enforce those: all you can do is go through the agency's internal complaint, grievance, and/or appeals procedures, unless you have a written empoymet contract including a union or collective bargaining contract: if you have such a contract, get in touch with your union for help.