Hatch time on Silkies??

[E.M. Silkies]

Does it usually take Silkies longer to hatch from the time they pip? I set RIRs and Silkies and 9 RIR's have hatched while none of my Silkies have. Is it usually a longer process for them? Or are RIR's just fast hatchers? This is my first time hatching eggs in an incubator so any input would be great!
Thanks all!

 

[E.M. Silkies]

Oh btw today is day 21.

 

[Elite Silkies]

I've had some hatch within a few hours after pipping and then some that have taken almost 24 hours. But, I don't think they are any different than any other breed.