I am one of the many that fell behind on my student loans

I am one of the many that fell behind on my student loans, and didn't qualify for an extended deferment or forbearance. I recently was contacted by prmr credit, set up a repayment plan, and have been making payments (direct debit from my checking account). I just received a letter that sallie mae is garnishing all of my state and federal tax returns in order to pay off my debt. Is this possible? How is it that I'm making payments, but am still getting garnished for the full amount I owed? Do you have any idea of what I should do? If sallie Mae wants to take my returns, that's fine, as it is their right to their money, and I fully understand that. However, should I not get my monies back from premier, who must not be paying sallie mae?