I guess the best thing I learned along the way was more ventilation, especially at the end. If you have near or fully formed chicks that did not hatch out likely your ventilation was not enough at the end. They really need the oxygen at that time, and in our efforts to raise the humidity we sometimes think it's right to reduce ventilation. Also the more water air is packing the less it's able to exchange gasses so more air has to pass by to get the same ventilation to the chick. So raise your humidity another way, just greater surface area of water mostly. Also watch out for your heat getting too high, the chicks are now generating their own heat so sometimes the temp in the bator spikes without it even trying to heat it up. Depends on how many eggs you have still in there. The last days are not a good time for the bator to be crowded! Too much heat, not enough ventilation.
If your using your own eggs make sure your hens are on a breeder ration, at least 18%, and have a vitamin mineral suplament so you have high hatchability. If your roo is playing favs with a few hens lock him up for a week then rotate him out with each hen, or a couple hens each day so they all get to be with him once every 4 days to a week, and he is alone at night. They don't need him every day, in fact a breeding stressed hen has lower fertility than one that is more left alone. They store his sperm for a long time, and he needs to be motivated to want all his girls if he is a picky type.
