
Just happened to me with my bitch, who had a litter to this young proven stud 15 months ago.
I have had bitches miss on 3 previous occaions, one twice, on her first mating with an older dog who was later found to be firing blanks. then after two litters she was mated to a maiden dog over severtal days, and missed, both the dog and she went on to have pups with other partners. Her daughter was tried with a older stud who had recently had a singleton pup by one bitch, and a month after she was mated another bitch had a single pup to him, but she missed, so failing fertility was our conclusion. Sometimes it is just poor timing or something not going quite to order in that cycle.
We are lucky our bitches are so fertile, as humans cycle every month, but women rarely fall pregnant at their first attempt.