Additional cost for missed flight

On the 22nd of April 2015, I missed my morning flight with Mango airlines from Joburg to Cape Town for work. I was then charged an extra cost of R597 which then had to be my own personal expense.<br> <br> My collegues went to Cape Town a couple of months later, missed their flights and had no additional cost.<br> <br> I would like to understand how this works as I cannot find any information on additional cost for a missed flight on the Mango website and would like a refund if the charge was not meant to be charged.