If there are no veins or movement by day 13, it is most likely a quitter. Also, if the air sac looks good (it should look separate from the rest of the egg, no blending into it) then it might still be developing. If you are still unsure about it, I would put the egg back under the hen, (I would never throw an egg I’m unsure about our, and most of the times a hen can tell if an egg is bad, and will take care of it herself) but candle it again on day 16/17. The egg should have lost weight by then, have a larger air sac, and the dark area should almost fill up the egg. Overall, though, I think that one is a quitter.