Silkie(h/h) X Silkie(h/h) will always produce Silkie(h/h).
Silkie(h/h) X Silkie/Split(H+/h), will produce both silkied, & Silkie splits(Smooth Feathered).
Silkie(h/h), X Normal smooth feathered bird, will produce Silkie Splits(H+/h).
So one parent wasn't a Silkie in the breeding.