Aww yeah sorry to hear that. Did it appear anything else was wrong with it?
I know I've read sometimes there is a reason they can't finish the job. =( Deformity, mal-position, etc. Plenty say don't assist, you may have had to cull it later or it pass a few days after, etc. You did a good job! And you got some wonderful babies to tend to! Focus on the positive!
I'm a bit torn myself at the moment. Woke up to 4 total hatched, all but 2 of the remaining 15 have teeny cracks. I know I have more time before I should consider assisting but I'm sure I will stress about it, since some are going well on 12+ hours from when they internally pipped.
I did read a several people say they had lower hatches when humidity was 70%+(possibility of chicks drowning during hatch) so I'm going to be honest I've fiddled with my "vent holes", also near where the vent holes are is a hot spot and the one thermometer therethat would read 100-102F when the other reads 98-99, there is a line of vent holes by the glass door and a place for humidity tray(I lost the tray, but put duct tape and a sponge there). And anytime(like hatching) that the humidity went over 70, opened those holes for a few minutes til it was down to 60-65(I also have another hygrometer/thermometer as well as a 3rd thermometer that isn't digital on the wall.
Most responded to tapping the glass and that very much encouraged the first baby so I've probably been doing it more than realistic but hey it worked. Who doesn't like some encouragement?