
I would say there are two possibilities -going from my own experiences. Either she had gone over on the second day and the dog knew it (3 litters is plenty, he is certainly experienced), or she wasn't actually ready yet. I have a few times had bitches where she was flagging and allowed the dog to mate her, and the dog did as he had the opportunity, but then the day after he'd no longer be interested. He'd then get interested again 3-4 days later. It's like the first mating was just for the heck of it. Last time it happened was with a litter born a year ago tomorrow. We had the one mating then nothing for 3 days -the dog was not interested despite the bitch being very flirty. Then he would suddenly mate again and we got two more ties. When the bitch was scanned the vet said she was fairly sure the second mating was the one that took rather then the first. Pups were born 62 days after second mating and my breed is usually early so I would find it hard to believe it was on day 66.
But yes, my bitches certainly flag before they are proper ready and so I don't mate the first day they flag. In fact one of my bitches was notorious for flirting and ended up tying with a neutered male of mine several days before she would allow an entire male to mate her. And yes I have had litters after a single mating.
I hope it's not famous last words as I have just mated a bitch, but so far I have never had a bitch that has been mated NOT fall pregnant -and I have never progesterone tested.