@Bluezoo Before eggs hatch, they sweat. Beads of fluid will leak through the shell. They can often end up looking pretty grey and soggy. Don't think they are dead just because they look so bad.
They will slit the egg, usually at one end but it could be at both ends and even in the middle. After slitting the egg, they usually rest for a day or more. The activity of slitting the egg is what triggers them to pull the egg yolk into their body. Before hatching, there is a tremendous amount of yolk still outside their body that must be pulled into their body and sealed off. They count on that egg yolk to sustain them for several days after hatching. The circulatory system that runs around the inside of the egg must also shut down and be clamped off. All this takes time, which is why you shouldn't interfere. You can bleed out a baby by cutting the shell before the circulatory system has closed off. Quite often the baby's head pokes out but they lie there with their eyes closed looking very dead. I've had babies that had their heads stuck out like that for a couple of days and they were just fine. In one case, the humidity of the container must have been too low as the shell started to harden again so I did have to cut the baby out but that was just for one clutch. Usually, the shell stays soft and leathery.
Your eggs look very round, but I've had eggs that were quite round hatch just fine. Do you have a lot of fruit fly cultures going? You'll need a LOT. Good luck.