Poor service regarding account and arrears no one can tell me where its coming from

Manquis finance has my account as in arrears. They cannot tell me where this arrears are coming from. After months they told me its a December payment that was missed. I have sent proof of payment for all months that i am with them but they always tell me the issue is escalated. No feedback no progress justs calls every moth telling me im in arrears and now they want no add an amount to my monthly payment. And every time i phone the person can only escalate the issue. After months of trying to sort this out I'm left with o other choice but to complain publicly.