Allocation of payments

I have made a payment on the 26th of August 2015 and Im still waiting for the payment to be allocated. I have e-mailed the proof of payment several times already, but I keep on getting sms' s to say my account is overdue. In the meantime RCS is charging me interest because of the payment thats outstanding according to them. This is totally unacceptable as my account have been paid long ago and now I get charged interest for nothing. I hope RCS is going to reverse that interest seeing that the fault is on their side. I have also made several calls to rectify this matter, but still nothing have been done.