Has your incubator been cleaned? Those things can be breeding grounds for bacteria and that bacteria CAN and DOES kill the eggs. Temp could be running too high or spiking and killing them. Keep a log and note details several times a day. Temps, humidity etc so you can trouble shoot what is going wrong. Also, are you sure they are dead? They sleep lots while they are developing inside the eggs. I have hatched out ALOT of eggs and it is quite uncommon to see them moving unless you catch them at the right time. They are growing and developing and that means they sleep alot!