
At what days in her season have you tried mating her. Sometimes you may not get a tie as the bitch isn't ready yet, or of course the dog may not have ejaculated.
For best chance of a conception you should mate her every second day for as long as she will stand. Once she is no longer willing this should signal that she has gone past the fertile stage.
The most common reason for a bitch to miss is mistiming the mating.
Some bitches ovulate earlyu in their cycle, and some can be very late.
It may be wortth getting a series of POrogesterone blood tests done to pin point ovulation, and time mating accordingly.
I know how frustrating this can be!