Experienced bitch won't mate?

My whippet bitch has had one successful litter. However, since then, although she will flirt with my stud dog, stand for him and allow him to mount, as soon as he gets anywhere near the right position she is off! This has happened for two seasons, so this time, thinking she might not want to mate with a dog she lives with, we brought in another stud and my boy went off on holiday. However, her behaviour is still the same. Any ideas?

Have you had her swabbed to be sure she is ready?

Thanks for the suggestion. She has had a boy with her throughout the entire season the last twice. I haven't had her swabbed because she is standing and clearly ready. She even comes up to the boy and invites his attentions in the most indelicate way! I am baffled because she was fine first time round and clearly loved motherhood.