Jumping in here with a similar situation. I'm fairly sure it's not due to poor diet. Here it is...
I purchased 5 Silverudd Blues from Greenfire hatch date was July 13th. Two of them died within the first 3 days. Of the three remaining, two of them lagged behind in growth and at four weeks old they were half the size of their hatch mate.
In August I purchased 6 more. Again from Greenfire with a hatch date of Aug 10. One of them died at 10 days old leaving me with five. Of those five, two of them are still tiny. By that I mean that they are not fully feathered and they are the size of about 1 1/2 week old chicks.
So my current flock of Siverudd Blues consist of 3 that are 9 1/2 weeks old and 5 that are 5 1/2 weeks old. I integrated them together last weekend and held my breath hoping no one would pick on the two tiny dots. To my amazement, not only did no one pick on them, they seemed to actually adopt them. My two tiny babies immediately started trying to crawl under the older birds for warmth and protection. Two of the three tolerated this and while they didn't comply with the babies requests, they did allow them to nestle up close. The funniest and sweetest part of this story is that both of the older birds are little cockerels... only the hen basically said no thank you!
Anyway, I diverge. The thing I wanted to point out here is that even though these guys are vastly different sizes, they got on just fine when we brought them together. Also, since both batches were fed the same food and some grew while others didn't, I don't think it was the food. The two from my July hatch date that were lagging have caught up with their hatch mate. You can see the various sizes in the pictures. Sorry the lighting is so bad. We keep the heat lamp going for the tiniest two because they are not fully feathered yet.
The 'dots' have started turning around since we integrated them with the older birds. It just seemed to liven them up and give them both a boost.
To be honest, last Saturday, one of them was so bad off that I really expected to find it pass away by the next morning. Instead, we brought in the older three and it just found it's "mamma" and came back to life.