15% protein explains it. That's meat bird feed, intended to control their insane growth rate so that they don't die before they are big enough to process. Normal chicken breeds need a minimum of 18% protein to support proper growth and development.
What was the protein content on the grower? Some grower feeds are specifically for meat birds, and don't have enough protein in it for most breeds. Layer feed is for actively laying birds only.
At 20 weeks old, they should be at their adult size.