* The female dog might not be in heat. A female dog is only fertile for a short period of time during her estrous cycle, called "heat". If mating occurs outside of this fertile period, pregnancy is highly unlikely.
* The male dog might be infertile. This could be due to various factors like age, health issues, or underlying medical conditions.
* The timing of the mating might be off. Even during a female dog's fertile period, the timing of the mating can be crucial. If the mating doesn't occur close to ovulation, pregnancy is less likely.
* The female dog might have a medical condition that prevents pregnancy. This could include issues with the uterus, ovaries, or other reproductive organs.
* Contraception. While not common, there are some methods of contraception available for dogs, including hormonal treatments and surgical sterilization.
It's important to note that even if a mating doesn't result in pregnancy, it can still be stressful for the female dog, especially if she's not used to being around males. It's best to keep dogs separated unless you're aiming for a litter of puppies.
If you're concerned about your dog's fertility or if you have any questions about mating, it's always best to consult with a veterinarian.