hi my collie bitch was mated by a proven stud dog but has not produced pups, this is the second time she has not caught, she has previously had 2 litters to the same dog, and both are in good health, the dog has covered and produced pups since so i know it is not him.
anyone any ideas?

It would take me ages to reply in depth as i am mobile, but look at my webs Thinking Of Breeding Your Dog? in one of the menu options is a section about fertility issues

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SOMETIMES YOUR KNIGHT IN SHINING ARMOUR IS MERELY AN IDIOT WRAPPED IN TINFOIL

Sorry, tiggie230 - not what you're wanting to read at all - but surely 2 litters produced by your bitch, especially to the same dog, is enough? What on earth is the point of 3 litters with exactly the same breeding? If your bitch is of good type and temperament and with good health test results, then surely it's better to mate her to different dogs (though I still think that three litters is quite a lot!) then at least you'd be adding something different to the breed's gene pool.