Different strokes for different folks, same with dogs I suppose, with their heat cycles.
If she is a maiden (first time breeding) that could be one reason why you might have a little trouble with the tie, but I think it's a safer bet that it is just a bit too soon. If you've got the stud there with you just keep trying ........when the time is right, and maybe with a little help from you, they will likely get a tie. Good luck!