Could it be the nutrition of the parents? Maybe they have slightly different needs than the other breeds?what if it’s a nutritional imbalance or something similar.
Years ago, I had a nutritional deficiency show up in rabbits, when I had not changed the feed at all. I eventually decided the manufacturer must have changed the recipe, because supplementing a particular thing made the rabbits go back to their previous level of health & reproduction (same exact rabbits: good results, bad results, supplement brought back good results. Slacking off the supplement brought bad results again.)
So maybe the feed changed, and is still adequate for the other breeds but not for the silkies and kikirikis?
I don't know what nutrient it might be, but maybe you could try either a general supplement or a different brand of feed, and give the parents a few weeks on it before collecting eggs for another hatch.