Money gone missing

I helped my disabled sister to change from the gold card to the black post bank card. Feb there was money missing an amount of R491.I have enquired with postbank to find out where the money has gone to. They have up until today not responded to any emails or phone calls. The agent I spoke to advised me that I did a cash back on the day the money was paid in. I however disputed this as I never did a cash back. I asked them to tell me the store i supposedly did the cash back at. I am still awaiting response they just ignore all emails sent to them, and just refused to give an answer .I have also enquired about the charges when the account was just opened it is now almost the end of April and i have not received no feedback from them since March when i sent the enquire. I would like to get some answers and proof of the money that was taken from the account, and the charges that was on the account but its a new account.