My June hatch was less than stellar, but part of that was the weakness of the eggs (I thought).
Anyway throughout the june hatch I used a combination of 5 thermometers. One that combined the thermometer and hygrometer (I think that is right -- to check humidity) that had a digital readout was my main one, but I had 3 little aquarium thermometers and another digital one. sometimes they all read the same, but mostly they varied. I relied on the digital one that also tested humidity mostly because it was lots easier to read.
I decided that before I subject any more eggs to my care, I was going to get a handle on what temps are really accurate. I use a glass of hot (106 degree) water and a human heath thermometer. I submerged all 3 of the aquarium thermometers in the water and left them for a couple of minutes to stabilize and stuck in the human thermometer. it beeped and I compared the temps without removing anything from the water. They were all within a half a degree of the same temp (by this time 103.7). I then compared the temp on one of the aquarium thermometers with the digital in the brooder and the digital I have in the incubator. I am not pleased. The one in the incubator reads 4 degrees hotter than the tested temperature of the little aquarium thermometer. The secondary digital one in the brooder with my 2 chicks reads nearly 5 degrees too cool (when compared to the tested temp on the aquarium one).
The long and short of it is that I was incubating at about 4 degrees cooler than i thought I was, and probably that was the problem with the hatch. Not much hatches well at 96.
Inexperience. I needed to slow down and do those tests first.