If it is being kept in the exact same conditions as the rest of the eggs and it isn't looking good then throw it away.
Fertile eggs will deflate and shrivel when the incubation medium is not moist enough. But like I said if you've done it right and all the other eggs are fine, it's likely infertile. Don't go and add tons of water to the medium now because too much water will cause the eggs to burst at some stage. Your water to medium ratio has to be just right.