"Fertile" eggs arent hatcing?

[bops]

I checked five eggs i got today, all five had that white circle on them. But not one egg has hatched with my last 3 hens who went broody ( 25 odd eggs or there abouts ) i only have hope for three eggs!
What other problems could be causing this?
Also if a silkie rooster has a chick with say a light sussex, would it take longer for the chick to hatch as Silkie eggs are much smaller? Just a hunch:/ hope so:/

 

[sourland]

All eggs will exhibit a white circle - it is the nucleus of the single cell. In a fertilized egg, the circle expands slightly larger in a bulls eye appearance. Do an on site search to see representation of the 'bulls eye'. If you have a silky rooster with large hens, there is a possibility that he is not making 'contact' even though he is going through the motions of mating.