Hi there! First let me say, sorry to hear about the DIS chick. It's hard to loose them even when they never hatched. As a hands on hatcher, (one that opens frequently during hatch and will even candle at hatch time occasionally) rest assured that the chick not making it had nothing to do with anything you did. 14/15 hatched chicks validates that you did an awesome job of incubating, so pat yourself on the back. I don't usually candle after day 18 until the end of the hatch when I have a break in the action and just have to know if any unpipped eggs are still viable. Last hatch, I had a couple that were about 24 hours behind and those poor chicks were probably yelling at me the whole time cause I think I candled them 4 or 5 times until they finally pipped...lol
The theory behind not opening the bator at hatch pertains to pipped eggs and generally if you have adequate humidity even then the chances of compromise are lower than people might have you believe. If it wasn't I'd never have the successful hatches that I do. The key is in the humidity.
Now in regards to the DIS chick, 100% hatches are not the norm simply because we can not in general give 100% of the eggs the optimal conditions that they need because of the individual variables in each egg. If I had to make a guess where this one is concerned, this would be conclusion:
Chick has absorbed yolk and is in postion. Fully developed, healthy looking. However, there appers to be quite bit of excess fluid from the egg. This would leave me to believe that because of shell quality, that particular egg did not loose sufficent moisture and that is what caused the death. Chicks get into position (head under wing- beak up) around day 17 and yolk absorption generally takes place day 19-20, so that would make me think that death occured late day 20-21 ish.
I would further surmise that this egg was less porous/thicker shelled because it did not loose the moisture as fast as the rest. I have also read that the darker eggs have a harder time loosing moisture, I believe, due to the pigmentation. I have even heard of some BCM hatchers "sanding" the shells to help with moisture loss.
I'm by far no expert, but in my experience, that is the conclusion I would draw for myself.
Congrats on the 14 hatchers. Awesome rate!