
It could just be that the primary urge just is not there - he does not know what to do because the instinct is missing - he just does not fancy bitches or may be nothing at all the urge to procreate has passed him by. It may be that one of his hormone producing glands are not working as they should and he does not have the balance of hormone needed to stimulate the urge to mate an in-season bitch.
Have you had him checked, is he fertile, does he have any health issues - did his first bitch snap at him, has any bitch snapped at him at any time, has he been punished or shouted at for mounting anything, even a cushion.
Think it may be best to stop trying to use this dog until he has had a vet investigate his inability, his total lack of natural urge. Nature sometimes knows best and there may be some good reason why he is not able to produce progeny and if he never has done so then he is not a stud and may never be - and it may be that it is for a very good reason.