Pay, the only reason Pandas breed so poorly in zoos outside of range countries and so well in china has hardly anything to do with husbandry or luck, it simply comes down to numbers. China has its prolific panda mills full of breeding animals. When a female cycles, there are males a plenty for her to breed with, and if they have a few that are non-breeders for whatever reason, no biggie, they have many others to take their place. With these loaned animals, it quite different, with all these single animals, they're absolutely hooped if something is wrong with one of their animals, especially the female. Look at poor Memphis for example. They've had their pandas for years, and are stuck with a dud female who cycles a-seasonally, and have had absolutely no luck getting cubs out of her. If China didn't keep this stupid monopoly on their animals, a new female could have been transferred from Atlanta or the Smithsonian, but no, instead they all have to go back to china and Memphis can get screwed. Of course the situation isn't so bad when the male is a behavioural non breeder, as AI seems to be rather successful with animals in good condition, but still...
And of course this is not to poopoo on San Diego at all, but more just a general frustration the whole panda situation.