the student loans I have with Navient

I followed the online instructions on how to submit a principle only payment for the student loans I have with Navient. I submitted a $50.00 check in the mail that clearly stated that this money was for principle only on the check. This payment was in addition to my required monthly payment. I checked my online account for payment/transaction history and found that it was applied to interest after the check had cleared. I feel as though this is highly unethical as I followed the instructions on the Navient website and I made this payment in addition to my monthly payment amount. I will continue to send in checks with a principle only payment in addition to my regular monthly payment and will make one complaint to the BBB per month until this unethical practice is corrected.