I said temp is the likely cause but the majority of those things are also very possible.
Improper handling or failure to fumigate an incubator can cause contamination that will seep into the pores. Could make every egg quit.
Unless one stores properly or keeps eggs over a week, eggs stored too long is a likely cause.
Flock diseases can go undiagnosed. Could make every egg quit.
Flock nutrition can be deficient. It takes better nutrition for breeders than that for general egg laying. Could make every egg quit.
Many people are too quick to medicate and pesticide exposure is always possible. Could make every egg quit.
People with small flocks without keeping breeding lines in separate quarters can easily have inbreeding issues. Could cause many quitters.
Unless one has a reliable turner, turning can be an issue.
Poor ventilation, unless addressed can also be a problem. Forgetting to remove air plugs, etc.. I've had eggs with sealed pores. After the first few days CO2 above 1% is a problem.
Hygrometers are notorious for inaccuracy so humidity can easily be a problem.
Embryologic development accidents can happen in any species at any time.
I'm just saying that there are so many things that can go wrong, telling someone it is always temperature may mislead them and lead to multiple failed hatches until all possibilities are considered.
Well, I feel I just made an innocent suggestion and usually the simple solutions are more likely. I do hope they figure it out, but I am in too good a mood to argue with you about someone else's eggs.