Website doesnt allow changes

We need to change our booking dates. When we try to do it on the website, a pop up screen just says sorry your not able to change your booking online. Why is this? As they clearly state that you are allowed to do it and save the additional call centre costs. <br> <br> We bought return flights (2 adults) and the dates to which we are changing to is at least R1 200 cheaper than our initial tickets, but sjoe do we have to be charged twice per each person for the change in dates, for departure and return, even if we bought a return ticket? <br> <br> Hope to hear from you soon