FNB doesn't want to reverse my money

Yesterday (23 Oct) I withrdrew R150 at an fnb atm and I only got R50. I immediately called fnb customer service and they said i should go to the branch to fill in a form so that they help me. I did exactly that and the lady told me that she sent the matter to the relevant department and they will contact me and reverse the money.

This morning I got an sms from fnb saying they can't help as the transaction was shown to be successful. So does this mean I lost my R100 and they can't do anything about it?

One guy who works next to the atm mentioned that I am not the first to experience that. My question is, why isn't fnb doing anything about that atm that keeps *****ing people's money?