I have some eggs getting ready to go into lockdown on sunday too. When I candle, first of all its really supper hard to see the chick. I have to really strain to see it moving but one thing I have noticed, the air cell has dipped. Have you noticed yours doing the same thing? dipped meaning the air cell space has grew.. Now when you say underdeveloped, what exactly are you seeing? can you see threw the egg? if so how are you seeing the chick moving. There shouldnt be any space from the membrane to where the chick is, it should be a really dark mass.. give them one more candling, take notice of the air cell and where the chick is it should be really dark..