It's pretty normal for few or none of a clutch to hatch. I've found eggs dead with turtles in all stages of development. Incubation drastically increases the likelyhood of more or all of the eggs hatching, though. I've missed nests in the yard before (in natural range for the turtles that laid them) and rarely do I find successful hatchlings, normally it's dead eggs at some stage in their development. Inside I have closer to a high 80's % rate, maybe even 90's.