Yes, they rest a lot before hatching. If something went wrong you would see a blood ring, or blob at bottom of egg.
Do you have a picture of an egg that would be dead around day 16-19 just so I can see what it would look like?
I've had early blood rings, but I've never seen one appear after day 15ish.
The last egg I had that died, never made it to the air sac, it was curled up, no veins. We just left it alone and waited for it to hatch, when we did the eggtopsy around day 27, there was nothing wrong with it, other than it just never got it's way into the airsac. Everything else looked perfect.