Breeding help

Hello again! I have a 8month old mini rex buck that came to us at 6months. He had a successful litter by the time he came to us two months ago. However, any time I try to breed him, he is completely uninterested and has recently opted to sleep instead. I also have another buck whom is 11months old, and he is interested in the doe but will mount and just sit there. All he does is mount the doe and sit like that while pulling the doe's fur. Any tips or knowledge as to why they do this?