female newfie wont accept the male

Question
my female newfie is almost three years old and my male newfie is two years old. He is very attentative to her and she will not accept him. they are both vet checked and healthy. SHe is supposed to go into heat the begining of september and he has already started to try and mount her. She gets aggressive with him and is that way all through her cycle. I am experienced with breeding (german shepards) and never had any problems. Do you have any suggestions??

Answer
She could be in a preheat.  If she's not allowing him to mount her, that means she's not ready to be mounted.  

What I would do is have a progesterone test done to see where she is and if in fact, she is having a false heat or pre heat.  If that's the case, separate them for a few days so that when it IS time for mating, he's not scared of her and she's not frustrated with him.