We are back from an unsuccessful try to mate our 5-year-old bich.  She has had two litters, both with very easy matings.

As always, I did Progesterone testings:

day 8:  2,16 ng/ml
day 10: 5,34 ng/ml
day 13: 16,88 ng/ml

We tried mating on day 11-13. She bit him away on all days. The stud (who has sired two litters) was less and less interested, on day 13 he hardly looked at her.

My question is:  Could it be, that day 11 was already too late for her?  Are there bitches whose ovulation occurs at rather low levels already?

Her last litter was conceived on day 9 (one mating only), after Progesteron of 5,1 ng/ml on the day before.

I was very puzzled by her behaviour and am trying to understand so that we will be more successful next time.

Sounds like she simply would not accept  this male.

A friend tried more than once to use a particular  male, but the butch would have none of him, but readily accepted a different  male, on both occasions resulting in a litter.

Can I jump in on this thread and ask is that the norm for testing - 3 times?
I had mine tested Friday ( also day 8 and was planning to go to-day and when I phoned my vet was not in to-day which I did not realise.
No one else there I would see!
Although she missed last time had earlier successful litter mating between days 15 to 18.

Basically you test every other day until you get an ovulation reading. As I knew my bitches did not show signs of being ready early I did first test day 12, then 14 with reading of imminent do retest next day, where got positive result.

This is why Progesterone testing can come out very pricey if you need a lot of tests,  but if your lucky you may only need two or three.