what are your incubator temps and humidity?? are you turning them right? most of the time, dying at a late stage means improper turning or humidity. people used to say "the more humidity the better" but that is actually the other way around. Now, its the less humidity the better (for incubation, not lockdown. during lockdown humidity must be at least 70%) now people are incubating at 40% and 30%. even 20% humidity and getting better hatches than 50% or 60% humidity. im doing 20% right now (this is my first time) so im hoping for all of mine to hatch.
or it might be a problem with the flock. disease, bacteria, stuff like that.