My bantam silkie and ameraucana always hatch on the 21st day too. If they are hatching earlier, it is typically due to incorrect thermometer readings and it is higher than it says.
My eggs aren't Silkies, they are just regular but I will tell you that I candled on day 4 and didn't see anything either. Then I candled on day 10 and WOW! I saw veins and chicks. Day 14 was even better. Be patient! The time will fly by! Tomorrow is day 18 for me and I can't believe it is already time to get ready for the babies to hatch! Hang in there!