Don't even get me started with this! I have two young males (2014) that will only pair with 1 of my 5 females. I've rotated them many times and they're only interested in the one girl. My amazing breeder male will breed anything that moves and has no problem with these girls, so I think it's the young males that are reluctant, maybe not the females.
One trick I just learned about that people swear by is putting the unbreeding pair into the cage of the breeding female. So remove the female that is breeding and put her in another tub, then put the male and female that wont breed into that breeding female's regular cage. This is supposed to trick the male into breeding with the female that he wouldn't breed with before by mixing up their scents or something. Now if it's the female that's denying the male, then I don't know what to do there.
As bcr229 mentioned, she may be too young.