Sorry, I don't think Miss Lydia's tag worked for me.
From the video, it definitely looks like she is having a hard time supporting her neck, that could be caused by a few reasons, but what we tend to see with birds around that age is a nutritional deficiency, often either in thiamine (b1) or vitamin E. I see that your giving her B vitamin and poultry nutri drench in her water, but I would personally suggest against that since the taste can be unpalatable and they're water intake might decrease. Instead try mixing some B complex, and one capsule of human 400iu soft gel into some mashed scrambled eggs, and see if she'll take to that. Provide clean water.
The lump is on the side of her neck, correct? How does it feel, like a bean bag, hard, or like air? Does it go away in the morning?
How big is their pen area, what is the temperature? What are you feeding them?