I don't candle until day 5-6 and I usually won't discard anything until almost two weeks in ā I think it's really tough to know if you have a problem until you're at least a week into the hatch, as I've had some hatches develop VERY fast and others be a bit sluggish.
For what it's worth, with shipped eggs I usually have at least a few that make it all the way to lockdown and never hatch. I've started opening them up to try to figure out what's going on, but honestly I haven't worked it out. Usually by hatch day the air cell is where it ought to be, although the chick is often not oriented properly toward it. With some hatches, I've used the turner (side-roller) immediately. With others, I've let them rest point-down for up to FIVE days before setting them in the turner. Doesn't seem to make much of a difference.
I have a new cabinet incubator that tilts the eggs back and forth but keeps the pointed end down, and I'm curious if this is going to help.