I had a blood ring on one of two eggs that my hen hatched a month ago. At the time I didn't know what it was, but she often moved nests and I had to place the eggs back under her. I read that too much movement can dislodge the blood vessels and cause it to die. I have another four my second broody hen is sitting on and she's staying put, but one of the eggs I candled today (day 7) looks like it has the same ring the previous egg that died had. So I will have to check again in a few days.
I think sometimes babies just don't make it.