Are you opening the 02 vent on top of your hatcher? I forgot once and all mine died in the shell from 02 deprivation. The temp, 02 and humidity has to be right. Good luckI have had hatching issues also this year. I am getting 95% to 100% development and then around day 18 to 21 they die in shell. The first one or two from the group will get out fine. The third doesn't fully absorb yolk, and either dies or has to be culled. The rest don't even pip.
I'm not sure what to make of it. Ron, I have just ordered two of the thermometers you recommended to check my Genesis. I had to replace it a couple of months ago. My first hatches of the year went great. I am now wondering if the thermostat/temperature control on the replacement top is off.