I'm wondering if you break them open to try and determine what went wrong. What do you look for, if you do? Are there problems you can diagnose by looking at the ones that don't quite make it? I don't mean rotten or undeveloped eggs, but the ones that develop to pip, or almost to pip, then just die. In my last batch, I had some that had been refrigerated, some washed, some not. I forgot to mark which was which until it was too late and they were mixed up. So some probably had a pretty slim chance to begin with. I had 5 not hatch, out of 18, developed far enough that gave them extra time to hatch, in case they were just slow. By yesterday, I knew which ones were dead, because they wouldn't hold heat. They had cool spots even when in the 'bator. Live ones are uniformly warm, because the chick's body generates heat.