The main causes with the humidity, is keeping it too high during incubation where condensation can build up in the air cell and when the chick gets ready to hatch and internally pips the air cell, it can drown, so you can find a fully developed chick that doesn't hatch. During lock-down high humidity isn't such a factor as then the chick needs more humidity so it can turn in it's shell after it has pipped and zip until it hatches out. This is just from my experience. I have experimented with various humidity readings until I got around 100% hatch rate and I have had some that have been 100% but generally I will have at least one egg that doesn't make it, a quitter.