Not able to change flight when flights are available online

Dear Safair, can you please explain to me how this works? I had a flight on Safair booked Jhb-Cpt @ 10:25 today, I arrived from Gabarone a bit earlier (@ 08:00) and I needed to get home urgently- I check online and there is a flight available Jhb-Cpt @ 09:45. I rush to your sales office at OR Tambo Terminal B, I ask the friendly lady - who helps me immediately- if I can possibly change my flight to the 09:45 one, the price of the flight was even the same (less) than what I paid for my original 10:25 flight. She replies ‘Sorry ALL flights are sold out for the day’ without even trying to help. I walk 5 m away from your sales office, I buy ANOTHER ticket for the 09:45 Jhb-Cpt Safair flight online and I check in. WHY?? Why was it not possible to change my flight and this trip which I needed urgently costed me R6000+ one way?