It sounds to me like you have everything right in the incubator. Your humidity is definitely NOT too high. It also sounds to me like water is not dripping on the eggs, but the eggs are sweating. This is usually seen on eggs that are going bad. Since all of the eggs are being exposed to the same environment, and not all of them are going downhill, then I would expect that it has nothing to do with your incubation methods. Sometimes eggs go bad, and can do so for various reasons. The embryos may have been weak for some reason and are dying. The eggshell is there to protect the embryos from invading bacteria. However, the eggshell are not airtight, as they do allow breathing and osmotic transfer of water in both directions. As a result of this, bacteria can get into the egg and kill the babies inside. That is another possibility. Some form of birth defect could be present, and that killed the baby,or even as some have thought that Homozygous Spiders were not possible due to it being a lethal homozygous, it could be something similar to that.
I don't know that any research has been done to provide info on all of the possible causes of dying eggs, but most of what I mention are theories developed by numerous breeders after experiencing this over the years, and doing their own investigations. I have been known to cut open dying eggs, slugs, infertile eggs, and any other strange egg that I don't believe will hatch, just to dig around in there, and see what I can learn. It sounds to me like you did nothing wrong. They are just not all destined to make it. That is one of the downsides to breeding.
Hope that helps,