It is random, top of the pile.
I applied and didn't hear back (good track record etc) for about 3 months whereas people who applied, numerous fails etc, got lucky, and heard the next day. (I was told there is a trend that if you apply late on a Thursday, you may hear on the friday..).
Finally got a call and a couple of emails, but have already started with another airline.
The adviceI hear from my fellow grads is to keep applying through SAA and get yourself to the top of the CV pile. (I only sent it in once, perhaps why I didn't hear back straight away)
Good luck.