How am I incubating my eggs wrong?

I have a Little Giant chicken egg incubator with the automatic egg turner, and everything works fine. But whenever I put a batch of eggs in they only develop to a certain point and then the chick dies and im wondering why that is? Because of the placement of the automatic egg turner i cant use the water holders in the bottom for humidity so i am forced to use 1/3 of a wet sponge. I just want to know what im doing wrong so i can hatch some eggs. I also have three thermometers that work fine so i know the temperature is ok. I just want to know whats going wrong and how to fix it.