Try putting a little sugar in the water. If she perks up a bit she might be interested in some scrambled egg or some hard boiled egg yolk. Also, plain yogurt is good.
Are they warm enough? They should be at 85-90F at 2 weeks.
It's a tough one. Look for ANY differences from where you are keeping the others- bedding, food molded, water from somewhere different, any tiny detail. And move the remainder even farther away, wash like crazy between handling the sick one and the others.
I don't know of anything you can do...