Help, Silkies die in lockdown.

[dtb2579]

I have hatched many eggs successfully and this year started to try to hatch Silkies. I have been adding other eggs to the batch too (silkie/cream legbar silkie/benevelder Benevelder amongst others).
The Silkie cross breeds and other various breeds pip and zip just how they should. Spot on 21 days almost to the hour but the pure breed silkies almost all have late deaths. My eggtopsies show either some yolk and some that have internally pipped and all yolk absorbed. Also the ones that do hatch have a hard time of it.
This latest batch I tried dry hatching and still the same outcome. I have sourced silkie eggs from different places through the post and collection.
My theory is the silkie eggs are smaller so more susceptible to changes in heat.

 

[ChickenCanoe]

It is true that the lower mass of smaller eggs will fluctuate in internal temperature more quickly if the incubator temperature doesn't maintain a steady temperature.
There may be a relationship but there may also be other things going on as well.
I would look at breeder age, inbreeding and perhaps even breeder nutrition.