Too high of humidity and the air cell will be too small to supply air during internal pip.
Too low temps during incubation and you will have improper embryo development..too weak to hatch.
Too high of temps in the hatcher and the chick will not be able to absorb their yolk to complete hatch.
Malformation or malposition of chick (head under wrong wing) and chick is unable to pip.
Poor breeder stock, poor nutrition of breeders, etc.
Old eggs - but these usually end up blood rings versus grown chicks that don't hatch.
Lethal genes can also cause dead in shell chicks.