Feathersexing is tremendously misunderstood on this site. It requires that the father be pure for "fast" feathering" and the mother "slow" feathering. That way, the males get a fast from their father and a dominant slow from their mother and will be slow feathering. The females get a fast from their father and nothing from their mother, so they will be fast feathering. But, just like red sex links, once that initial cross is made to produce sex links, it is done. You cannot use the next generation and get sex links.
If you look at the bottom of Tadkerson's first post on this thread, you can see Tadkerson's write-up about it.
Tadkersons Sex Link Thread
I've seen many posts on this forum where people claim to be able to sex chicks of certain breeds by their feathers at hatch. I've yet to see an explanation of why that works. I am constantly surprised in chicken genetics because there is often something else causing an effect other than the simple straight-forward explanation, and that may again be true in this case. But, if that is the case, it is not the feathersexing gene doing it. Until someone can explain what what the genetic mechanism is causing that, I am going to remain extremely skeptical that it works, other than when the father is pure for fast and the mother for slow.