Hatching call ducks

[Laloni]

I have a question about hatching call ducks I had nine eggs they were all fertile had babies halfway through most of them died One made it to the very end pipped through and then died I cracked it open to see what was wrong with it and it has a yellow sack on the outside of its stomach between its back legs I've had this happen before and they always die any ideas what this is and why they're doing this

 

[JacinLarkwell]

That yellow sac was the yolk. It was still absorbing it. How high is your humidity?