SSA garnished my payments for overpayments that were not my fault. After the overpayment waiver was granted, they refuse to refund the money they've taken. What is my course of action?

This is for Supplemental Security Income (SSI). They're saying it's a "need based program" and that they won't refund the money, but I received a letter from them several months ago saying they were holding money until the waiver was approved. Is there any legal action or a complaint process I can take that will garner me results, or should I just eat the loss?