For starters, you should always hold your female during the mating/tie. Never let them just do it on their own.
If she just went into heat a week ago, it could be that she's just not ready yet. Have you had progesterone testing done to determine when her most fertile days are? Always a good idea for beginners and when you're not sure.
Also, many females are simply freaked out by the whole process the first time (or two, or three...) around.
I am assuming with this being your first time breeding, you have an experienced breeding mentor that you are working with? (If not, then stop until you do have one!) Have him/her come over and observe their behavior together and assist you. That is what they're there for.
Good luck and welcome to YT!