She probably has a respiratory disease. I'd purchase some powdered oxytetracycline (Terramycin, Duramycin, Tetroxy HCA-280, etc.) and begin treating all of your chicks. You can usually find the oxytetracycline at a livestock supply store for a relatively inexpensive price (9-15 dollars). The dosage varies depending on the product; I do know that the tetroxy dosage is 1/2 teaspoon per quart of drinking water given for 7-14 days. Do not give dairy products or probiotics during treatment.
I'd also get some chick vitamins/electrolytes and put them in the chicks' water to boost their strength. Make sure they know how to eat and drink properly; if they don't, encourage them to by feeding scrambled eggs, applesauce, or moistened chick feed. Dip their beaks in the water to show them where it is.