Pay interest

I am using this forum as I find the call centre very frustrating. My 98 year old mother has an FNB credit card which is paid by debit order every month. For the last two months she has not been debited and is scared they will charge interest. Yesterday the call centre gave the reason that in September my mother had a credit (an incorrect credit instituted by FNB which was reversed) Apparently if the computer reads a credit they do not activate the debit order. In October there was a correct credit being overcharging of fees. Once again n o debit order. These credits did not exceed the amount owing and my mother is fearful that interest will now be added. This is NOT a threat. It is a statement. If there is interest which the call centre said there would not be, I cannot call FNB. I will hand this over to the banking ombudsman. After being a customer for over 50 years, when sorted we have no option but to consider using another bank's facilities.