I'm wondering if you're seeing independent movement from the embryo. Sometimes when an embryo quits during the second half of incubation, it will move around as you twist the egg to candle. I've been fooled by this "movement" a couple times! So check to see if it's moving while you're holding the egg very still. It may just be a little behind or about to quit, if the veins are receding. It's hard to tell sometimes with shipped eggs and they tend to quit later on just as much as in the beginning. Don't give up hope in it yet. If you candle on day 19 check and see if that air cell draws down. Draw down doesn't happen on an egg that has quit. So what day is day 21?