
The most common reason for failed conception is mating at the wrong time. This is often because the bitch has ovulated unusually early or late, or that she has not synchronised her physical receptiveness with what is going on inside, so is standing before or after whe is fertile.
If this is the first time she has missed I would advise having a series of premate blood tests done next time, bearing in mind that the eggs are ready for fertilization two days after ovulation. I know some people have rushed to mate the bitch when the tests have said ovulation is imminnet. My own bitch had imminebt results 3 days running, and finally a result that said she was or had just ovulated on the fourth test. That was day 12, and she wouldn't stand until day 16 when she was going over, the result a smallish litter of 4 live and one stillborn, born at 59 days from this mating, proving that the premate test was right.