Millie Fleur Chicks Died Yesterday - Help?

I brought home 2 Millie Fleur Chicks 3 weeks ago... they didn't seem to be growing at all, but otherwise seemed healthy. They were in the brooder with a Maran that was one week older, and a Barred Rock and Americana that were 3 weeks older. When they all initially went into the brooder, there wasn't a massive size difference... just a bit. But now the other chicks are getting huge, and the Millies still weren't growing. See pic below. I took that photo last night when the first Millie had passed, but the other was still alive. The second one died overnight. But I'm telling you, other than not really gaining size, she seemed healthy and was running around the brooder and pecking at the ground before I went to bed.

Then, both little Millies died last night. I've never once seen any of the other chicks pecking at them. On the contrary, the Barred Rock and Americana always let the little ones snuggle underneath their wings... all the time. I just don't know what went wrong. Help? Ideas? I'm so sad.