How many times to try mating our female

We have an 8 year old female, who had a vet assisted birth on 4 August 2010. We have now mated her 3 times to our stud male - who is a proven stud and has successfully mated with our 3 other girls, but still no spit off. As it's getting late in the year, would it be advisable to leave her now until next spring, or should we keep trying. We will be bringing Pepsi down again in the spring to cover another couple of girls who are due to birth April/May. OR should we get the vet to scan her to see if she's pregnant anyway?

I would have her scanned as not all females will spit off the male when they are pregnant. We had one of our stud males mount a pregnant female just 6 weeks before she gave birth and she quite happily cushed down for him. The spit off method is not really that relaiable for establishing pregnancy.

Hope this helps.

Kind Regards, Mike L