unable to change date of flight

I was at the capetown airport. I couldn't board my flight which was departing at 13:50 because I was at checking at 13:25. The flight closed at 13:10. I had to then go to the client services counter to change my flight dates rather than loosing the rand value already paid. I was told that I could change my departure flight for later on that day only and not to another date. I could not change the flight date to another date. They did allow me to change my return flight date and I paid the flight difference. But they refused to change my departure flight date. They insisted I had to travel on that day on a later flight and pay the price variance but not another day.