Misplacement of a payment a client made

I made payment of R700 on the 31st November 2017 on my account but the payment was not picked up. This is the account I have always used for the pass ten years and they never missed my payments I have always made. I don't know their system, i don't have access to their system to do that , they surely have someone that receipts payments from their remittance advice and trace the payment if they can't find it. They keep sending me to and from the bank. The bank has already provided me with what they need. They won't do more. It is really getting frustrating as debtors are calling me for collection almost everyday for the same thing, i'm really tired. I'm thanking you in advace for your assistance in the matter.