She might just be a bad setter, then. I had one hen who never managed to cover all of her eggs--even when she was down to four, one somehow ended up kicked to the back of the nest. (I felt bad for her, so I picked up some feedstore chicks. She turned out to be an excellent mother.)
She may also have taken a longer beak than usual to go eat. If it's cold where you are, that could have really hurt them.
My other guess has to do with nutrition. Higher nutrient feed (chick, allflock, game) has a strong correlation with chick/egg vitality.
In the end, I doubt you can be sure. I hope your remaining eggs hatch.