Every month Navient tries to contact me

Every month Navient tries to contact me and my grandmother regarding my student loan payments. But here is the issue I pay them online every month and they continue to keep calling me. Then they call my grandmother and threaten to take away her home and everything she ownes because I am not current on my payments. But that is incorrect. I fax them my confirmation to show that a payment has been made on my loan payment and they still keep calling, this happens every month. When I do speak to the Sallie Mae collectors I tell them that I made my payment online and then they check it and then say “Yes I do see a payment has been made.” Well why couldn’t they check that before they call me or my grandmother? Considering I make my payments online every month!