Lost a cheque deposited at forex branch

I deposited a cheque at the FNB forex branch in Sandton City. The cheque reflected in my account on the 10th of April and on the 21st of April, the money left my account for no apparent reason. Through several conversations with call centre agents I have managed to establish that the cheque was lost. However, it has been over a month now and I am unable to find out why/how it was lost or to resolve the situation. Call centre agents promise to call me back and don't. I fail to understand why I should suffer the financial consequences of FNB losing my cheque. I really need this money! HELP!!!