When you candle your eggs right before lockdown, what do your air cells look like? They should take up about a third of the total space in the egg--they will seem really, really huge if they're big enough. If they're smaller than that, the baby can drown after pipping, because it can't hold its little beak up high enough to get enough air.
If your air cells are not developed enough, then you are running your humidity too high. I have had much better results with my hatches since I started hatching dry. I hatch duck eggs, and I don't add water AT ALL to the incubator for the first 25 days (ducks take 28 to incubate). This does not work for everyone--there are many factors to calculate--but if you are losing most of your babies right at the end, I would strongly suspect drowning.
This unfortunately has happened to me with my most recent hatch. I am running staggered hatches and had a batch of quail in there. The current batch of eggs went through two different lockdowns with the high humidity before their own lockdown, and the air cells never developed properly. I had one little duckling pip and then expand the pip very, very quickly, trying to get up above the liquid so it could breathe. I feel terrible that I didn't help it, but I didn't realize what was happening until it was too late. Others just didn't pip externally at all, though I know they pipped internally. I did hatch about half of them, but I'm accustomed to much higher percentages when my humidity is low.
Good luck. It's a tough learning curve, and I'm sorry you're having so much trouble. I hope one way or another you're able to fix things and get some good hatches. Your one cute little guy must be a consolation anyway, and think of it this way--babies raised in very small batches tend to be friendlier than those with lots of comrades.
