By the time that a male dog actually ties to the female, he will have already ejaculated sperm filled fluid into her.
People prefer a breeding tie as a tie decreases the amount of fluid that is able to leak back out of the female, and thereby gives a higher chance of the female getting pregnant.
However, not obtaining a tie, does not mean that there will not be puppies from the dogs breeding.
The most common reason for a tie not being obtained is due to the male’s bulbus glandis swelling outside of the female, rather than inside, which can sometimes happen when the female does not stand still long enough.
When dog breeding, problems arise when an owner of the female decides that a slip mating will not result in puppies and rather than waiting to see, the female is taken to another stud dog.