Overcharging on card account fees

I have a Discovery platinum card and secondary card. When I received my March 2016 statement, the card fees appeared to be high so I called the call centre to query this. The consultant then reviewed my account and confirmed that I had been overcharged for a 3rd card (a second secondary card) for several months. He explained that I would receive a credit for this and the card charge would then be rectified going forward. When I received my April 2016 statement, I saw that nothing had been done as I am still being overcharged for the card fees and have not received the promised credit. This whole experience has made me question what the benefit of a platinum card is because all I can see is the cost being higher but not receiving any added value at all. It's certainly not better service, that's for sure. I'm going to seriously weigh up my options but in the meantime require that I at least be credited for being overcharged all these months and for my card fees to be accurate going forward.