Why won't my dogs tie when they are mating?

[Scott]

I have a sydney sillky b*tch that is on her 9th day in heat and she has been standing since her 4th day, the dog I'm using is only 8 months and is really keen but I'm not seeming to get a tie, they do it and then he goes away till his snake goes back into its cave and then he isn't inerested anymore. Why won't they tie? is he not getting it in the right place? Sorry I know it's kinda a gross question.
Its kinda stupid when you people think your acheiving something by being abusive! Think you guy have no idea, lol....