57 Southern white rhinoceros have been bred at Whipsnade Zoo between 1971 and 2008. I've seen this trend at a few zoos. A zoo is one of the first holders in the region and breeds multiple offspring over the first few decades; then in present day, there are many holders and they breed as sporadically as everyone else - while still riding the achievement of what they bred decades ago. Adelaide Zoo do it with Asian small-clawed otter; Perth Zoo with Sumatran orangutan; Taronga Zoo with Sumatran tiger and Nepalese red panda.
Looking at the four females in their present herd - Clara (1981) has produced three calves to date - 2000, 2005 and 2008; while Mikumi (1990) has produced four calves in 1997, 2000, 2001 and 2005. Clara (aged 39 years) is getting on in her years (though dams as old as 42 are recorded in the studbook); but Mikumi and the two younger females - aged 22 and 13 should yield better results. Their bull is even recorded in the studbook as one of the most prolific breeders (having sired 15 calves), so we know he's not the issue. Perhaps the younger cows are suffering from estrus suppression from Clara (the dominant female)?