I have been watching your natural nest hatch and it was amazing.
Not posting there since I got Jumped on that thread...
Regarding the two that did not hatch, there is no such thing as an egg shell too hard for a chick to hatch from.
The three causes of late term death are, in order:
1. Temperatures too low or too high
2. Humidity too high or too low during incubation
3. Ventilation
Those three things account for nearly all of the causes.
Ventilation is very critical after the first week or so.