Possibly genetic, or result of brain damage/infection, or lack of certain vitamins
The easiest way to test (with out vets) is to syringe feed, and keep isolated . After a week of water, vitamins and food through syringe they may show signs of recovery, if the problem is the lack of vitamins.
If they have not started showing signs of improvement, then it is most likely genetic or brain damage and the kindest outcome is to put down the chicken as if it is genetic or braindamage, it is incurable and only gets worse causing the chook pain and distress.
hope this may shed a little light,