Cat, it’s not always the rooster’s fault when they are blank. Some are more fertile than others and some get less “active” as they get older. A rooster may be injured or sick somehow, so he is not very fertile. Frostbite to the comb and wattles might cause a drop in fertility. If the rooster is molting he may not be very fertile. An immature rooster may not have earned the respect of older hens so they don’t allow him privileges. So yes it is certainly possible it could be his fault.
It could have to do with collecting and storing the eggs. If they get too cold or too hot it’s not good. Them cycling between cool and warm a few times is not good even if it doesn’t get that extreme. I had a lousy hatch one time, 10 out of 30, because I shook them up when I was bringing them home on a bumpy country road. I did not cushion them enough. Boy was I mad at myself! Health, nutrition, and age of the flock can make a difference. Hens tend to lose fertility when they get old, even if they still lay eggs.
Now I’m ready to throw you a curve ball since you mentioned cochin. Some thick feathered chickens, like orpington or cochin, may have feathers so thick the rooster can’t hit the target. This is normally not hatchery quality birds but rather the birds bred for show since they are specially bred and fed to have thick pretty fluffy feathers. It’s not unusual for breeders of show birds to trim the feathers around the vents of the roosters and hens to make that target more available. Most just take scissors and trim the fluff away. One lady said she plucked them so they will grow back before the molt.
I don’t know what is going on with yours. Good luck on that hatch.
