Looking at your incubator my concern it your heat source. You have what looks like a radiant heater with no fan at the top and it is pointed towards the eggs. I see the fan at the bottom, but it suggest to me you are cooking the eggs.
If you have ever stood in front of a wood, gas or electric stove then you feel that radiant heat and your skin gets real hot/warm on which ever side is facing the heat source...but the side of your body facing away from the heat source is does not. You need heat but not direct radiant heat, heat the air not the egg and the rest will follow.
(like a hot sunny day on a bald head, it burns)
A quick remedy would be to put a metal plate in front of the heater (you''ll have to rig it) to deflect the radiant heat waves, turn the fan on. The better Idea would to be: to have the heater and fan pointing to a dead wall and the air flow making a circular like circulation without the fan/air hitting the eggs directly.
I could not tell much about how you wired it up, but the fan should always run, and the heat kicking on only when the the temp needs to be raised.
If it is not working your round disk may need to be replaced, the can go bad get a hole in them as they are soldered. If a pin hole is there, you can try to fix it or get another disk.
buried some where in BYC is my DYI 48 qrt igloo cooler build, but its for quail and pheasant....but I turn manually by hand. Still, my point is I piped the heat to the bottom using a aluminum gutter pipe (insulated it). The heat source is up high but isolated and duct to the bottom, the blower fan pushes the air through to the bottom, Heated air rises, and the fan blower / air never directly hits the eggs.
Hope this helps.
