My Bitch Won't Breed My Stud!?!?!

I have a 2 1/2 year old bitch and a 1 1/2 year old stud that I purchased with an intent to breed them. I kept them apart every time my bitch went into heat until this time I wanted her to get pregnant. The problem that I have been having is that she has gone into heat and won't let my stud breed her. Every time he tries to mount her she just sits down or rolls over. However we also have a FIXED male mutt that I just now noticed she keeps trying to get to mount her. She will go up to him and push him around and stick her butt in his face until he mounts her and she will let him but as soon as my stud tries to mount her she just sits down again. What can I do??? Also, I have tried keeping the stud and bitch seperated alone but the just lay around and she shows no interest in breeding until she is around the other male... Will she eventually let my stud breed her?? She was just bleeding 2 days ago...
   
Thanks for any help!
   
Isaac

I doubt it. Sometimes dogs are like that, she may be trying to breed with your fixed dog because he has less of a hormone smell, weird as that may sound.
   
I'd recommend getting them fixed, the earlier you do the healthier it is, if they're not going to breed no point keeping them unhealthily unfixed!
   
Honestly, there are so many dogs int he world being euthanized, I know its wonderful and it really does make a lot of $, but is it worth pushing other dogs out of potential homes that will then end up at shelters?

Just giving an update...
   
Yesterday shortly after posting the message I took our fixed male over to my mother in laws for a few days so that there would be no interference and she was bred by my stud male last night! I guess for some reason she just saw our fixed male as the dominant male or something which doesn't make sense due to the fact that he is scared to death of anything including being pet because he was an obvious abused pup that happened to wonder up on our porch one day and my stud male constantly rolls him over and stands over him showing his dominance.... Oh well it all worked out!
   
Thanks anyway!
   
The enclosed pic is one of my bitch with my stud when he was a puppy!