Taken from the link I posted about making an air safety hole if you need to.
It is in this phase of hatching that many chicks die resulting in late dead in shell. It is a time of great stress and physiological change within the chicks body. The heart is pumping rapidly due to exertion and trying to compensate for changing blood gases. It is my opinion that inadequate moisture loss during incubation causes the chick and its supporting cardio-vascular system to be overloaded in fluid. With the heart having to pump faster and harder to compensate the chick goes into heart failure. The tissues in the body become swollen with excess fluid (oedema) and the chick weakens. The space for manoeuvring into its hatching position becomes even tighter and the chicks body is simply too weak to withstand the vital changes required. It is now clear why monitoring egg weight loss is so important!
I also incubate exotic and rare species of birds and every chick is vital. Therefore if I am concerned in any way about the chick or external pipping is delayed then I intervene. Using a sterilised tiny sharp drill bit I carefully enter the air cell at the centre and very top of the egg. I check using the candler that the chick is not directly below the proposed point of entry. By twisting the drill bit by hand the egg shell is gradually eroded away and a hole approx. 2-3mm diameter is made. This safety hole provides access to fresh air and needs to be no larger or premature drying of the membrane will occur. I have called this artificial external pipping. This safety hole has saved the lives of many healthy chicks over the years. I can recall instances of baby parrots successfully external pipping then going into rotation within the egg until their body occluded the external pip area and then died! Very annoying and upsetting to be so close to a successful hatching!