well first off, Is the male significantly smaller then the female??? If so, the male might be intimidated by her and he may feel like he is inferior so he won't try to mate with her. Mating in leo's is not a pleasant thing to watch, the male is ussually VERY aggressive, and will bite the female, sometimes even making her bleed. How did you incubate the eggs?? If only one egg was laid, she is probably a first time breeder, so it wouldn't come as a suprise to be infertile.