How old are your girls ?
If it's first heat (which most advise not breeding on first heat) likely it will not take as for the body isnt ready, they are far too young. Second heat you can't stop them usually
Or Perhaps you're too early on the cycle...
females heat cycle, which lasts about 21 days:
The first signs are swelling of the vulva, and usually several days later, a blood-tinged discharge. This first stage is called proestrus (or preheat) and lasts about 9 days after the blood discharge begins. Males are attracted to the female, but she is not yet interested in breeding.
The second stage, called estrus, starts around the 9th day of the heat and continues for the next 7 to 10 days, during which time ovulation occurs. Now the female will allow the male to breed her. Many females continue bleeding during this stage, in others the bleeding or discharge may slow down or stop. The most suitable time for mating is from the 9th to the 12th day.
Keep a close check on the female to determine when she is ready. Besides observing the vulva sponginess and discharge color, you can lightly touch the vulva with your index finger. If the female whips her tail to one side, she is probably ready to be bred. The best indication is the females behavior with the stud dog. She will not stand for breeding if she is not ready, and most experienced stud dogs are not seriously interested in a female that will not stand.