Firstmark Services Loans review: This should be illegal, it is stealing.

Firstmark Services claims to have not received payments on loans. When I provide proof that they received the payment, they apply it as of the date they resolved the discrepancy instead of the date the payment was made and the check cashed by them. This is not the first time they have done this to me so I am hoping someone can help with this. They are stealing money from me by applying less to principal and more to interest. Please help.