Haha I had to look it up, too:
"A
balut or balot is a developing duck embryo that is boiled alive and eaten in the shell. It is commonly sold as streetfood in the Philippines."
I don't think that was your intention.
Usually with chicks if humidity is the problem they reach full size and then either drown at hatch or just don't hatch- but the yolk sac will be absorbed. Like I said i don't have any experience with ducks but I would guess not a humidity problem.
I found this chart useful for chickens. Most is probably also true for ducks.
http://msucares.com/poultry/reproductions/trouble.html
It lists the following as most common causes for early embryo death.
Improper incubation temperatures (usually too high)
Improper egg turning Turn at least 3 times daily.
Inherited low hatchability Avoid cross breeding. May need to secure different breeding stock.
Improper ventilation Increase ventilation rate in incubator and/or room, but avoid drafts. Add oxygen at high altitudes.
Pullorum disease or other salmonelloses Use eggs from disease-free sources. Have NPIP representatives blood-test the breeder flock.
Improper nutrition of breeders Provide a well-balanced nutritional diet to breeders.