What sounds like is happening is you received a batch of chicks that came from pullets that are just starting to lay. The chicks develp leg problems early and have a high mortality rate. This is something that the hatchery isn't really going to know as more than likely they didn't raise the broiler breeders themselves they just bought the eggs from a producer, the eggs however will be small and noticable to the hatchery. What hatchery did you use? I've had this happen a few times with only certain hatcheries, it seems like they have a reputation for using cheap eggs from pullets going into lay.
See the eggs are still good but they don't hatch well, so the hatcheries that choose to buy them get a heck of a discount. They figure if they get a 40% hatch... they are doing great. As long as they eggs are cheap enough, the downfall is the chicks die easier, don't preform near as well.... or give you chicks like you have.
One thing that I can tell you to try and save the rest is to get some beef liver and cut them up for the birds to eat. This is high in riboflavin and will help them develop their joints and legs. One other thing that may help is to drop the protein to 18-20% to slow their growth down... feed them 12 hours straight (or just daylight). When you restrict their feed like your doing they may not be getting enough to eat, this schedule may have worked for your healthy chicks that you had previously but these chicks need the food.
Do you have any pictures of the brooder and chicks? This may help fixing the problem.
So again... beef liver... lower protein... and 12 hours of feed.
Good luck.
