
She could be completely ready, but not conceive, even though both she and the dog are tested fertile. :( I had a bitch like that - I mated her on three successive seasons to the dog of my choice, and she was a total t*rt each time - loved every minute of it! But she missed every time, even though he was siring litters with other bitches. I mated her to his full brother, and we got a litter straight away.
Sometimes individuals are incompatible.