Chick won’t grow

This is Tiny. She’s a 6 week old Cuckoo Maran, along side her sister who we got at the same time. Tiny as you can see, is not hitting the same miles stones and appears to not have grown much bigger than when we first got her. We’ve been cycling nutridrench, probiotics, scrambled eggs… nothing seems to promote any growth. Has anyone else ran into this issue and what did you do? We are going to start the integration process slowly in a couple of weeks, and we don’t even want to risk her because she will inevitably get picked on due to size.