Do you have photos of your hen and her poop?
When was her last egg?
Any bloat or fluid in the abdomen?
Keep her eating/drinking, see that her crop is emptying overnight. Do look for lice and mites as well.
Hard to know what's happening with her. I'd keep up with the vitamins - make sure they contain E, B1 (Thiamine) and B2 (Riboflavin). If they don't, then you can use human vitamins give her 1/2 tablet B-Complex and 400IU Vitamin E.
She may be having some reproductive issues or this could be disease like Marek's, unless you discover some type of injury. Just guesses on my part, at 2yrs old and no additions to the flock, I'd lean toward reproductive or possibly fatty liver disease.