Then I would guess that they have a vitamin deficiency of some sort that is causing weak legs. You will still need to wrap the legs. But I would add a good poultry supplement to their water. Something with a lot of the B's and E vitamins. Hard boiled chopped eggs contain all these and more, all the building blocks of life. The beauty of eggs is it's a good emergency food for chicks and can be fed for a short extended period of time without compromising the fact that they will eat less chick starter. I would feed egg a few times day, only what they will eat in 5 mins. Clean up with they don't eat, don't leave egg in the brooder under all that heat as it will grow bacteria over time. But definitely find a good supplement to add, sounds like they have deficiencies.