Lot of time seramas die at day 18 and sounds like to much humidity. Its goo when they peck and drown from the goo because to much humidity. So sorry!
For you hatching pros I have a couple of questions: I had two eggs that had pipped with large size holes. yesterday was day 20 so I went to bed and when i woke up they had died. I wasnt worried because they still had a day and were getting plenty of air...any thoughts.
also have you ever had chicks hatch that have a glue substance in there that keeps them stuck in. It reminds me of rubber cement. They chicks are all hard and crunchy when I pull them out and gluey in the shell. I have had this happen a couple of times and they have to get helped out of their shells.