@Parklandchicks, I made a homemade incubator out of a light bulb (with dimmer), file cabinet, jars of water and tin foil. We use a hygrometer and thermometer to monitor the environment.
I have no problems with the humidity, and we record temperatures which hold constant but I think the proximity to the light bulb is my issue. Some eggs inherently are closer to the light bulb, As we rotate the eggs it changes which eggs are closest. I want to say that the eggs closest to the wall cool more than they should and are not at the optimum temperature. On my next go I am insulating the unit more and adding a fan in side to ensure that all the air is one temperature. With still air incubators the temperature is never uniform so this is where I think I need to tweak. I threw out all the eggs today. It is day 27.