What do you feed?
I'd continue with the vitamins. Get extra protein into her like egg, tuna or meat.
Marek's is a possibility in chickens, so without testing it can't be ruled out completely. To me, she moves like she's been injured.
Give her 1 tablet Calcium Citrate daily for a week. You can find Calcium Citrate with D3 in the vitamin aisle of stores like Walmart, CVS, etc. Give 1 tablet daily, just pull down on her wattles, pop the tablet into the beak and let her swallow.
You dusted for mites - does she have mites or...