Cooking oil.
Some people like to use the spraying can, but I use the liquid, scoop it up into my hand, and douse the areas. The cooking spray does do a real good job getting the legs and feet if they get leg mites. Thank goodness I haven't had that problem.
I do all around the chicken..vent of course. Side of vent, and up into the top just above the tail, and then beside the wings, on the back of the neck, and the chest. Really gets them fast. They don't have anywhere to go. The oil isn't hurting them on the skin. In fact, after a DE treatment, and I have to go ahead and do this, I can see how dry their skin is. I just tell them they are getting a nice little oil treatment for their dry skin.
Shinx, check your Rock out for mites...you probably already have, but you would be surprised, at how much they can take out of a chicken. Maybe it's something like that, just a coincidence that she is not doing well, and it isn't the same thing? Those mites can multiply quick, and I'm finding that DE just doesn't do the trick every time. Just a thought? Good Luck!