Sometimes the veins hide good, I've had a couple eggs hatch that I was never able to see good viens candling them. However the other majority of time of the time, if I don't see veins, the eggs go bad. I always give them a chance though just for those couple times.

Unless mold starting growing on them, temps got screwy, or the eggs dried out, theres not much more you can control to screw up the process, much of what goes on is out of your hands. Can't be getting depressed, it's going to happen, part of the hobby.