What went wrong? 40 viable eggs going into lockdown, only 14 hatched!

[McChooky]

A good broody hen is worth her weight in gold. I have had some superstars in my chicken raising history. Had one hen that I witnessed tank down two different hawks that were after her chicks. She could hatch 12 for 12 and raise them all.

This is the only broody hen I raised myself but when we was growing up I witnessed how good leghorns are too

 

[Project Blue]

Another thing that could have happened is there wasn't enough ventilation in the incubator leading to them suffocating. Once the chicks start internally pipping and breathing air I would think they'd need more available O2. If the incubator is packed full then this is pretty important. But if this were the case I feel some of the chicks would have died after hatching too so I can't say for sure. A lot of incubators have adjustable vents you can open or close to help with this. Mine doesn't but it is forced air and I often check the eggs daily so they get plenty of fresh air. Is your forced air (has a fan)?

I think your advice is spot on, and observant, especially the shell color/thickness. Against normal advice I open a small peep hole with late hatchers and orient them upwards to prevent suffocating in fluid as the early hatchers spin the late eggs and allow for more 02. This way you can remove the dead eggs, and live chicks, gooey shells so the live developing eggs breathe better and have less bacteria to deal with. Looking through the peep hole you can see the progress of those eggs, some need more development time, some might need help hatching with stuck membranes etc. If you work in a warm room and never let the incubator drop more than a degree and replace the humidity with warm water and add a little hydrogen peroxide to counter all the previous chick hatching mess I think way more chicks hatch.