
Usually because she isn't ready. My bitches will hump each other whilst in season but if one of the males dare look side ways at them they will be given short shrift.
Is it your own male you are using? If so then be guided by how they react to each other, my lads will check them out with a sniff in the passing and only get serious about mating when the time is right.
Have you been keeping notes on how she has reacted in previous seasons to get a guide as to when she would be ready when you decided to mate her?
I hope both dogs have had all the breed specific tests done and got good results before having pups.