Here in California, where the weather is nice all year round. I've been growing Koi for 7 years now, although my fish are healthy and large, I still have not had a spawning. I have heard that my supplier may sell sterile Koi. Has anyone else ever heard of such practice?
I have never heard of sterilizing a fish, but it could be in response to the ever growing ban on keeping koi. When I first heard about it, I couldn't quite believe it, but then we got a news clipping of a fine levied on koi keepers. Evidently the fear is that the koi will get loose and crowd out native species. I'm not sure how little bitty fish would be sterilized, but there could be a way.
Another possibility is that your fish are indeed spawning, but momma and daddy are eating their young.