
Genmerally to be sure of hitting the right time the bitch should be mated for as long as she will stand, in practical terms alternate days until she says no. This may make working out dates a little harder, but in a bitch that has previously missed it is best to maximise your chances.
Interestingly I recently had a bitch here for mating to a dog I bred. She had twice been to a dog and he had been rather uninterested in her and then cfouldn't manage to mate her, but she stood from day 10 or 11.
The same happened here, and on day 15 I had the Vet look at hger as she was very hard and unyielding in the area concened and the vet confirmed that she oculd not be mated by a dog in that condition, and unless things softened no dog could.
She was in fact mated day 16 to 18, and we are waiting to have pregnancy confirmed (or not). What I am trying to say by this is that the bitch was wiling to stand far to early sending everyone into a panic.