Mating is a natural activity in dogs. While bitches can only mate during their heat period, studs are always ready for mating. However, some studs are unable to mate due to different reasons which could be a primary problem with the stud or secondary to the bitch. 

Here are some reasons why your stud may not be mating

1. Lack of interest: This occurs rarely but common with inexperienced males who have not mated before. Apprehension and stress associated with mating can cause disinterest in a first-time male. To prevent this problem, it is advisable to have the two dogs meet before the mating day and owners should avoid stress or undue interference during mating. 

2. Mounting Problems: Once a stud shows interest in a ready bitch, the next process is mounting. The most common problem associated with mounting is a difference in size. Size compatibility is important for successful mating. If the stud is much bigger than his partner or smaller, there will be difficulty in mating. Furthermore, stress and anxiety can also prevent mounting.

3. Problem during the tie: Copulatory tie is the peak of the sexual process in dogs and it is the stage where ejaculation takes place. Some studs are unable to finish the tie due to reasons such as interruption during a sexual act, inexperience, fear and anxiety, or slip mating.

The above points summarized the reasons why your stud may not be mating. Beyond these, you might need to discuss your stud’s case with your veterinarian who will be able to help your dog if he is intended for breeding.