
Simply sometimes bitches miss. Of my own girls the first had two litters, no misses.
Her daughter missed on her first mating, had two litters, missed on an attempted third, but a year later had a third litter.
That ones daughter has had 3 litters no misses.
Her daughter had slip matings and no litter, next time she had a litter.
When tried of a second litter refused the dog point balnk, and next season with him staying with us has had a nice litter.
With bitches we are lucky that they do produce litters more often than not. A woman would not even be concerned if she had not conmceived for say 6 months, and a doctor woudln't be worrying until a year of trying.