How long is the cooling period and when? I have always left the cover off for a few minutes each day (or every other) to really get the air circulating and fresh. Until this hatch, everything went well so I don't think that's the problem. Instinctively I think a little fresh air is good.
And yes, this was a Marans egg. Of 10 that went into lockdown, only 4 made it. And one of those was "J". My last marans hatch saw 2 of 5 hatch. They were the ones who this time didn't even turn in their shell and had that awful jelly everywhere, though fully grown. So I think dry incubation is very important for them.
For me, I have to add water to one whole tunnel-thingy in the bottom of the Hova bator to have the humidity at 35-40%. I think the key for me is not necessarily to make the humidity SUPER high at the end, but rather to just behave myself and not open the lid at all.
Thanks for the support everyone - I LOVE BYC!