We're strictly talking about stories from 2012 though so I don't think that they should be mentioned.
Northern and Southern White Rhinos are likely separated by over a million years of evolutionary divergence according to (Groves et al. 2010). So it would be erroneous to assume that just because Southern White Rhinos breed so well that therefore Northern Whites must too (although it may be the case that they do). Just look at species from other genera within the same genus as each other which exhibit differential breeding success from each other. But exactly why this is is still very much a mystery AFAIK. One could conjecture but without solid facts its pointless in my opinion.
Although, the conditions under which the animals are kept (including their diet) is one obvious variable which can influence the desire to mate and perhaps also fertility, though I'm not sure about the latter.
There has been no Northern White Rhino breeding since 2000, although mating has occurred earlier this year, which will hopefully lead to a pregnancy:
Hope for the Northern White Rhinos as Suni and Najin Mate; Twice! | Ol Pejeta Conservancy
Reference:
Groves CP, Fernando P, Robovský J (2010) The Sixth Rhino: A Taxonomic Re-Assessment of the Critically Endangered Northern White Rhinoceros. PLoS ONE 5(4): e9703. doi:10.1371/journal.pone.0009703