You get the odd dog that just knows and shows no interest in stock. I had one once he hit two nearly and hardly saw a sheep then one day lost a hare and had to come back through a flock and never even glanced at them. He just new his vocation in life and was bomb proof.
On the flip side my present dog which is a bull cross I have spent so much time around sheep its ridiculous and we got there in the end but I will never trust him. It detracts from him big time because you just can't relax but we cope.