a couple things come to mind when I read this...
1. humidity might not be high enough and baby cant pip internally
2. very well could be lethal gene..
3. I totally had a 3rd but now im in a writers block and cant remember... OH! thats it.. Serama eggs are frigile to being with so it could be literally anything!!
Anyways.. what type of incubator do you use? If you use hovabator or something similar, a friend and I have noticed better hatch rates when we took the plastic tray out of the bottom and just filled the bottom of the bator with water up to the wire grid just before the water touches the eggs! also we dont mark our eggs "X" or "O" we just place them and then roll our hand over them 3x a day! It has been said on the SCNA forum that maybe by marking "x" and "o" we are turning the eggs in too much of a ratio where as a hen turns the eggs a little over 100x a day little bits at a time.. not half the egg morning, half the egg noon, half the egg night.. Also if you use an egg turner... their bad for Serama eggs!