First, do not hatch eggs from a hen that lays double yolked eggs. This is a problem with the egg laying tract--you do not want to encourage this in your flock. Repeat: double yolked happen because there is something wrong with the hen.All of my chicks are hatched in an incubator. I do have some trouble with Betty and double yolkers. She is usually very fertile. I do have some chick's that develop and do not pip. This is a malposition problem. Most likely due to the humidity being incorrect. I only have one humidifier on the 18+ day eggs. The other hatcher just has water sitting in the base.
Second, Nearly all chicks that die on day 18 and 19 will be malpositioned. This is because they did not move into position yet. Malpositioned chicks happen later in incubation--like on day 21.
Third, Humidity is not the most common cause of day 18 death. Temperature is. Unless you are incubating outside of the safe zone for humidity, 25% to 60% then humidity is not the problem. Wet sticky chicks are cause by a development problem and Temperature needs to be controlled within plus and minus 1%.
Try this next time:
Do not set from the hen that lays double yolks
Get a Brinsea spot check and run the incubator at a verified 99.9F
Add a cool down cycle from day 8 to 18. Two hours with the lid off each day or if your incubator has a cool down cycle, run it for two hours a day.
Make sure turning is on and working--at least 4 times a day, 45 degree tilt.
Also, make sure that the flock does not have parasytes--get some rooster booster and start feeding them some mana pro.