They won't be dying on you, that's for sure. Just give them time to catch up and they will eventually get to the right size. Long about 12 wks will be the time to butcher them. You can increase proteins by mixing half whole grains and half grower or layer rations if you want to do a final push for weight and size.
The last batch you raised, were they fed only corn, wheat and oats or did you feed a higher protein feed then? If so, that feed difference could explain the slower growth...but slower growth is not always a bad thing, as long as they reach the size they are supposed to eventually reach. If it takes longer, so be it...the meat will taste better and they will survive to the end of the prescribed time and that's the goal of growing out slower.
Helped a lady butcher her CX raised on free range and FF this summer and she had the biggest CX I've ever seen...each were approx. 15 lb live weight and all healthy as oxen. Free ranged right up to the end all over a couple of acres of land. Their organs were all wonderful, they had enough fat under the skin for good eating but not any huge fat layers in the carcass. Just good, huge, healthy birds all coming to the finish. She fed organic layer rations and whole grains in her FF.
