I agree with those that say the plastic box - became the incubator, which it always does if you keep trying to "fix" what went wrong with the box and hatch rates.
And saying you're LOOKING at an unhatched chick to SEE what went wrong... ummm that isn't going to yield a LOT of info unless you're a scientist or vet and have lab equipment. LOOK it's dead. Barring any obvious deformity caused by EITHER genetics or your equipment, you aren't going to know what's wrong, only about when it happened. And you can't tell looking at it if it WAS genetics or your equipment - so NO information at all really except it's dead. That's not an experiment. You can't get any data except time of death and deformity or not.
I don't think unborn chicks suffer but I think playing around with what doesn't work is wasteful. Zero to 30% hatch rate, time and time again, is beating a dead horse to no good end. It's not simple, it's wasteful.
People streamlined incubator construction to make it reliable and constructive and useful - simple.
Progress happened for a reason. Used properly good and great hatch rates happen. Less waste. Less death.
Simple.