Awul service from Marquis finance

I purchased a vechile through Marquis finance in April last year - I asked to be debited on the 25th of each month - my account was debited at the end of the month and then not at all i have sent countless emails - to no avail i made manual payments eventhough i accepted the debi check mandate with my bank i get debited on the wrong days after making the payment and now i receive an email saying i am in arrears and i have spent hours on the phone trying too resolve the query and its affecting my credit score - I am in the process of securing home finance.