If you've ruled out leg and foot injuries, that leaves avian viruses or vitamin deficiencies.
Marek's is a common virus that affects the legs and causes lameness. Avian leukemias do, too.
But it's wise to try vitamins before you jump to conclusions it's a dreaded virus.
First try vitamin E (400) and selenium (25mg) If that's the cause, she should respond pretty quickly, within a week. If that doesn't work, try B complex. Same thing. it should work within a week or not at all.
Have you had any chickens die for mysterious reasons in the past? If the vitamin therapy doesn't work, you may be dealing with a contagious virus. If this hen should die, it's crucial you get a necropsy.