Sponges work to increase humidity as would some cotton cloth. Surface area rather than quantity is what raises it.
Another suggestion is to weigh eggs rather than measure humidity. Ambient humidity varies a lot as does the porosity of eggs. Eggs should lose about 13% weight during incubation. I usually use a pocket gram scale. But if you incubate a lot of eggs you can use a larger scale and weigh them all at once. However, I've found the random kitchen scale to not be very accurate.
https://poultrykeeper.com/incubating-and-hatching-eggs/weight-loss-method-forl-incubation/
Now to address your original question. I haven't seen it mentioned but the nutrition of the breeder flock will dramatically affect hatchability. The late deaths and other embryo issues would point me in that direction.
What do you feed your flock/s? It helps a great deal to improve the nutrition beginning about 3 weeks prior to collecting eggs for hatching.
Another suggestion is to weigh eggs rather than measure humidity. Ambient humidity varies a lot as does the porosity of eggs. Eggs should lose about 13% weight during incubation. I usually use a pocket gram scale. But if you incubate a lot of eggs you can use a larger scale and weigh them all at once. However, I've found the random kitchen scale to not be very accurate.
https://poultrykeeper.com/incubating-and-hatching-eggs/weight-loss-method-forl-incubation/
Now to address your original question. I haven't seen it mentioned but the nutrition of the breeder flock will dramatically affect hatchability. The late deaths and other embryo issues would point me in that direction.
What do you feed your flock/s? It helps a great deal to improve the nutrition beginning about 3 weeks prior to collecting eggs for hatching.
