Others have weighed in on diet.
I would immediately start feeding her layer mash with a couple of vitamin B capsules (for people from the pharmacy) mixed in.
Do this for at least two weeks before giving up.
How old is she, and can you feel any injury in her leg?
Two things spring to mind: Vitamin B deficiency (you can give her supplements from the pharmacy - it takes a week or more to take effect) or Marek’s (which sadly you can’t do much about).
No harm in trying vitamin B treatment.