I have just encountered this, thought I would pass it on.
we mix our feed for our chickens. its 25 lbs 18% layer crumble, 25 lbs cracked corn, some grit, some oyster shell, about 6 lbs of calf manna, and 2-6 lbs of black oil sunflower seed. we noticed this year our fertility was lower, and eggs weren't as big. after watching the birds eat, I realized they weren't eating most of the layer crumble, they did eat all the corn, all the sunflower, only some were eating calf manna. taking the layer crumble and some of the calf manna out of the equation lowered our protein content. I'm not a nutritionist, Ive been hatching for 3 years, so all I can give you is my experience. I am also finding an absorption problem with my feed. apple cider vinegar helps with the absorbency problem, add a tablespoon per gallon of water 1 day each week.
I used cat food to boost the protein temporarily, but there are things in cat food that aren't good for chickens.
we are now starting to lacto-ferment our feed, I don't know what the outcome will be. by research it should be way better.
if your eggs are large, but fertility is low; separate your rooster, throw him a hand full of cat food for a couple of days. let the roosters and hens out in fresh grass and I think you will see a big difference in fertility.
here are a couple other things you should know:
if you give a young cockerel layer crumble as the sole source of his feed, he can become sterile.
sorghum seed in your mix will cause more broody hens, and a higher outcome of pullet chicks.
never set eggs that are more than 50% dirty.
open that incubator 15 minutes a day. (simulates hen getting off the eggs)
make sure to sterilize your incubator between hatches.
for me the best humidity settings are 40-45% for 18 days, then raise it to 55-60%. you can actually drown chicks with to much humidity.
another way to think about hatching- the hen does not have a thermometer, a hygrometer, or steady temperature around her. she still hatches eggs. if the temp drops a couple of degrees for a day, don't worry the eggs will hatch. when I used little giant foam incubators, I done better without a hygrometer than I did with one. I would either fill the two line reservoirs or the circle, until day 18 then I would fill all 3.