He may be too young or he may be emotionally intimidated by the female or she might not be ready to breed. Dogs in season have specific points in their heat when they are willing to breed and the males can detect that based on the female's odor.
But at this age he should have hit puberty and be able physically to breed.
Try giving them some privacy to play and get to know each other and don't loom over them while they do this. Keep an eye on things but from a little distance.
It does help if you work on training the stud dog to permit handling during breeding. You can try the dogs together twice a day to try and hit the right time for the female. Usually the females are pretty forward with the males when they are ready.
If you know for sure the female is ready (progesterone testing or knowing she previously mated on a certain day of her heat) then you may need to do an artificial insemination if you want to use this stud for this litter.
If you have other males who are in sight of or hearing of the breeding a young pup can be intimidated by senior studs into behaving himself even if alone with a female.
If you aren't sure she's ready let them spend some time together and wait to see if nature takes its course. Praise him for any mounting behaviors or for sniffing her. Let him know that's what you want him to do.
Be ready to intervene and assist so no one gets hurt when he ties with her.
Hope this helps you!
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