I have a Pit bull I'm trying to breed?

I got a female pit.. she's small and I can't get them to mate... I brought her to his turf... and she nips a little and runs off from him.. he mounted her but it's not in the right place... I don't want to direct it cause who wants to touch dogs privates.... It's late in her days too.... hmmm any remarks.... thier both beautiful dogs... and totally health... and I bought my male for 500 but he is worth 15oo because he is blue... the female is tiger strip.... she is great.. just a lot smaller than he is...