Ah yes, the frustrations of living in a country which does not require all aircraft to be fully ADSB equipped. In your instance your Mom's flight was most likely done on an older aircraft type that is transponder equipped but that does not send out all of the required data for plotting on FR24.
With aeries that are full ADSB, they send out all position and aircraft data which is received by the ground receivers and then plotted on a map which we see on FR24. This is usually for all the major airlines and newish aircraft. On the older aircraft we can still see their positions but FR24 then uses Mlat to triangulate the position of the aircraft and then plot that on the map. However, in these instances you only pick them up above 10,000ft and you need a number of receivers in an area to triangulate the plot.
However, this is all changing as new aircraft right down to Slings and helicopters are now coming out standard with full ADSB. That is why you will see some of these on FR24 puttering around Gauteng and the likes. Ideally we would like to see map plots of every single aircraft much like you do in the northern hemisphere were everything that flies must by ADSB equipped.
Getting back to your Mom' flight, I used the playback option on the website to see what aircraft were departing Durban round about that time. It looks like your Mom's flight was being done by Safairs ZS-JRL which is a B734, an older model aircraft which does not have full ADSB. However, the flight was picked up on Mlat as per the picture below. Shout if you need assistance when using FR24.