I have just spent 3 days in the Sandveld during which time I visited 11 nests. The eagles there seem to be quite productive:
6 - incubating
1 - possibly incubating but not confirmed
2 - chick
2 - Black eagles present but not seen on the nest.
In comparison; in the Cederberg 5 out of 18 nests are being incubated and 2 have chicks.
There are lots of reason which could be played with - in the Sandveld I was mostly told where active nests are by Lucia Rodrigues. Whereas I found about half of the nests in the Cederberg by searching appropriate cliffs so perhaps in some cases I have found old nest structures. However, having said this, most of these sites normally have eagles in the nest vicinity, or in some cases standing on the nest or building it or even mating nearby.
In the Sandveld the nest spacing seems to be further apart than in the Cederberg... but this might change with more nest searches in both areas.
This is all preliminary, but there's some numbers, thought and questions...