Installment

I ve been battling to sort my acc.I paid 10 days late as I only then received my salary...which I told them.But still phoned like 7 times a day. So I paid a few days late more than my installment. But they allocated the payment for August already and now saying I m July s installment inarrears. How de hell does that work.We are still in July the logic would be deduct the amount of July ...but no they took it for August now I m still inarrears because of this..and no one seems to care to rectify this amount and allocate it to this months installment.