Well that's not good! I guess you candled them long ago & know they're fertile. Any way one of your brood could have bumped the Temp and/or RH out of whack?
The sensor for the heater is stapled to a plastic piece of flat paper like material. When my kid was rotating the eggs, there was a couple day period where it wasn't flat on the ground under the eggs - so it's possible the heater cooked the eggs, or, the opposite, our power went out for a few hours and the eggs got *really* cold for a few hours - then heated back up. So I don't know :/ I took a couple of viable eggs out and stuck them in my smaller incubator, but I had geese eggs in there, so only a couple could fit with them.