Oh, dear. No, it is not normal for the veins to disappear. Was your temperature off? At day 6, you'd only just be starting to see veins. Its possible you should try again tonight, in full dark, and maybe you can see more. Sometimes its hard to see during the day. Check again....
I could REALLY see the veins on these guys yesterday. Here's a bad picture of one of the eggs but you get the idea:
