My hatchery stock silkie is completely silkied except for her primary flight feathers (pictured here on yours) and 3 tail feathers, which for whatever reason are hard. Truly soft, whispy silkiness seems to be something that has to be strived for hard, like the pillowy cushions of a cochin. I think that perhaps without diligent selection in stock, you'll still get feathers that don't zip! Hatcheries aren't particularly known for their show quality stock since they have to do a fair amount of quantity > quality, so that may be what's going on here. If a bunch of them have single combs I think that's def. grounds to demand answers.