Something went really wrong in this last batch

Sounds like something was off with humidity and or the temperature. Did you happen to take pictures of the chicks that did not make it to the hatch? I'm not sure but I'm going to guess maybe humidity was a little high so they couldn't breath when it was time to turn in the shell to get ready to pip ( air cells to small) Or perhaps humidity was a little low and they got ( I forgot what people call it )... The membrane dried out too much and shrink-wrapped (that's what you call it shrink-wrapped) the chicks in the eggs. Try to figure out what the problem was and correct it don't automatically assist hatching... You really shouldn't need to assist hatching if the conditions are proper...
https://www.backyardchickens.com/t/878773/attention-newbies-to-incubating
This was helpful
https://www.backyardchickens.com/a/step-by-step-guide-to-assisted-hatching
So was this

 

Mississippi State Incubation Troubleshooting
http://msucares.com/poultry/reproductions/trouble.html

Illinois Incubation troubleshooting
http://urbanext.illinois.edu/eggs/res24-00.html

I find these sites helpful in determining what went wrong. As you can see, there can be a lot of different causes. It’s not always easy to figure out what happened. At least you looked at them so you have a starting point. If you had not opened them you really would not have had a clue.

Even if you think you are doing things identical in different incubations, it doesn’t always work out that way. There can be differences in the eggs going in or the temperature or moisture level of the outside air going in can be different. Hatching the eggs is as much an art as a science.

Read through those links and see what you think, you may see something that I don’t know, but the three things I’d think about are:

1) Humidity during incubation. If it is too low they can shrink-wrap. If it is too high they can be oversized and soft, called sticky chick.
2) Did they have good air circulation late in the hatch. They need fresh air to breathe, just like we do. They breathe through the porous shell. If the air plugs were in or something like that then they may have suffocated.
3) Were the eggs pointy side down? If the eggs are up-side-down they can’t internal pip properly and drown.

Good luck with it. These things are not always easy to figure out. And I’ve had hatches like that, some really good and some really bad. You can just hope to learn and get better.

 

Awesome links ridgerunner ( I saved them )