Here are a couple of troubleshooting sites that can help you narrow it down. As you can easily see, there are a lot of possible causes for an egg to stop developing at any stage. It’s not really easy to pin down the exact cause, but unless you open them and try to see their development like you did you really don’t have a clue. At least you have something to start with.
Mississippi State Incubation Troubleshooting
http://msucares.com/poultry/reproductions/trouble.html
Illinois Incubation troubleshooting
http://urbanext.illinois.edu/eggs/res24-00.html
In real general terms, if an egg doesn’t develop at all or dies in the first week of incubation, it is probably due to something that happened before the egg went into the incubator. That may be fertility, how or how long they were stored or handled, health or nutrition of the parent flock, or many other things. If they die in the last week of incubation, it is generally due to something that did or did not happen in the incubator, maybe heat, humidity, turning, positioning, ventilation, or cleanliness. Of course there are exceptions to this. There are always exceptions.
If an egg is fertile when it is laid the embryo is alive. So fertility is a big question for the ones that never developed to start with. But the embryo may have died after the egg was laid. Several things can kill the embryo. I once had 20 out of 30 local eggs fail to start developing. I think I shook them up too much getting them home over a rough country road. If you are having a lot of clears from local eggs, look at your storage and handling methods.
You can see the possible causes for eggs that fully develop but don’t hatch or even pip. I’ll go over some of the things high on my list but it is an incomplete list. Did you turn them, especially the first couple of weeks? Turning helps body parts form in the right places and keeps the yolk or developing chick from touching the inside of the shell and getting stuck or drying out. After a couple of weeks the body parts have formed and a membrane forms around the developing chick to protect it from touching the sides so after a couple of weeks turning really isn’t important but for the first week or two it is.
If you incubate them upside down, the chick may fully develop but is not in a position to pip.
If the incubator or the eggs are not pretty clean bacteria can grow inside the egg and kill the chick. But since you didn’t have any stinkers, this is not your problem. I only mention it to discount it.
At the start the developing chick doesn’t need fresh air. But as it develops it needs to exchange bad air through the porous egg shell for good air. If you have all the plugs in and don’t have good air exchange, the chicks can die before pip, after pip, or even after hatch.
Your incubating temperature can affect the hatch tremendously. If it is too high they can be early, too low and they can be late. If it is too extreme they can just die. Were the chicks that hatched early or late? Early chicks tend to be smaller. Late chicks tend to be soft and mushy.
Humidity during incubation and hatch is quite important. If the humidity is too high, not enough moisture evaporates so the chick doesn’t have enough oxygen in the air cell to stay alive between internal and external pip. These chicks are often soft and mushy too. If the humidity is too low during incubation the membrane around the chick, the one that protects it from contacting the inside of the shell, can dry out and shrink wrap the chick even before pip. The same thing can happen after external pip if the moisture is too low during hatch, but it can happen before pip too. Was that membrane dried out and collapsed around the chick when you opened them?
Different people mean different things when they say “dry hatch”. Some people don’t add water at all, some only during lockdown, and some mean they keep the humidity pretty low but may add water during regular incubation if it is really dry in there. Different people need different humidities, maybe even the same people but at different times of the year. There are a lot of different things that enter into that. A lot depends on the temperature and humidity of the air going into your incubator, but other sneaky things come into play too. Some people do really well with different dry incubation methods, some have disasters. This is one part of what you are doing that I’d really look at, but it could be something else entirely.
With those rates, something is wrong. You can and will do better than that. It’s not always easy, there is often some trial and error involved. But by opening the eggs and trying to gather information from that, you can at least make educated guesses. I wish you luck!
