My dog's first litter was very successful. I've been trying to breed her again, but she keeps twitching and avoiding the tie. What should I do?

HI

I have a 4 year old female boxer who had a litter of 9 pups at 2 years old. All lived, she was a great mom. I left her for 2 heats and then tried breeding her again with the same proven male. She was eager, but seemed to twitch to the side to avoid tying every time the tie was imminent. At her next heat the same thing happened. The first time we visited the male for 5 days in a row and the second time I had the male at our place. They were given time away from each other both times, and I had them together on what should have been her fertile days. I tried a third time, but had all the testing done at the vets to be sure we were catching her at the right time. She still is what I call a twitcher and there was no tie. I really want another litter from her as I want one of her pups. Any suggestions?

Thanks,
Lisa