Chicks can sometimes pip into a blood vessel. If it's only a smear of blood, not flowing or spreading, it should close up on its own. What you're seeing is the chick getting its first gulps of real air; don't worry if it takes a while between pip and zip, the baby is likely just resting. I'd be careful checking on them too much (for your sanity), especially if you have to open the incubator to get a good look. If one has been pipped for over a day, or some still haven't pipped days after they're due, then it's okay to start wondering.