That might be why the guy I get my buck from likes the two week minimum. That way if the first heat Isn't fertile he will still be there for the second. Not sure. I just went with what he said because he's been breeding and raising goats for over 20 years.
Maybe try to keep them together for a week or 2. The times I've bred mine the guy who owns the buck likes to leave him there for a minimum of two weeks. i've never tried to breed for just one day