Chicks That Never Hatch? Why??

[eggscetera]

Perhaps this has been covered before. This is my first season incubating eggs. I have a great incubator and have had lots of chicks hatch this spring (about 40 or so). My last batch just finished incubating. I had quite a few duds. I've not learned how to candle so I just put in all the eggs and hope most of them hatch. Anyway, after giving them an extra week (hope springs eternal) I decided that if they haven't hatched in 30 days they aren't going to (which is an entirely separate question- how long do you wait until you call it quits? 22 days? 24 days??)
So- I smashed the duds. Out of 10 smashed 7 had perfectly formed chicks. Why didn't these hatch? I suppose there are various reasons but I'm shocked at how many went most of the way and never even broke the shell. Any guesses as to why these chicks died before hatching?

 

[Ridgerunner]

You might look through this. Florida had a good incubation troubleshooting article too but they updated their site and now I can't find it.

Sme of these things are more for professional commercial operations so be a little careful how you read it, but most are applicable to us. You'll see that many things are not necessarily in our direct control.

Mississippi State Incubation Troubleshooting
http://www.poultry.msstate.edu/extension/pdf/troubleshooting_incubation.pdf

 

[eggscetera]

Oh this was very helpful. Thanks for the link. I have a feeling it was the humidity- for my next hatch I'll be much more careful to monitor the humidity. Thanks!