When you say temp is constant.... how old are they and how warm is the area they are in? They need a light. They need enough room to get away from the light in case they feel hot.
That being said... I had one set of guineas I hatched out, they all died one by one like that. The eggs had been very dirty, with many rotten ones I had to remove during incubation. So my natural conclusion is that the keets that had made it had gotten a bacterial yolk sac infection from the soiling on the eggs, which leads to a slow death by about 1 week.
The temp is 90-95•, they have a light, they have room in the brooder they dye when they are 3-4 days old! Only 1 in about 10 dye. Guinea eggs are dirty! so, I wash them and disinfect them before putting them in the incubator . I keep the bator disinfected at all times! I hatch out in a hatcher which I also disinfect.