If your candling showed good air sacs then humidity wasn't the issue. Without sufficient air sac they can drown when trying to internally pip. Your hatch time wasn't too late though I'd use the same thermometer next round and up the temp to 102-102.5F measuring same place you did this hatch. But all in all things don't sound way off. Two things that retard the absorption of yolk is low temp and high humidity. I like 30-35% humidity and always perform a salt test to calibrate hygrometer at start of each hatching season. Minor adjustments should improve your hatch (lower humidity to day 18 and bump heat 0.5F) but these can't be the real culprit to your poor hatch.
Shipped eggs? Classic most eggs develop and only a few hatching happens all the time with shipped eggs. It's a bit heart breaking as you spend money then get really hopeful with all the progress when candling then meh.
If not shipped eggs then vitality of parent stock might be in question. Line breeding to inbreeding over time will seriously reduce the vitality of offspring. Resulting in poor hatch rates. The other thing is nutritional value of feed given to parent stock. But too much inbreeding is usually the case in these two scenarios.
Edited by Egghead_Jr - 10/17/15 at 4:16am