Hi there! It’s my first time incubating eggs and we still have a lot to learn. We have nine eggs in the incubator. All nine are fertile. We are on day 7. For 5 of them, we can see the little body perfectly clearly and all the veins surrounding it. But for the rest (4) the veins are looking great but The heart is not visible and instead looks like a pocket of air. Anyone know what’s going on?