Welcome to BYc. Can you get some Poultry NutriDrench and give a drop to the other chicks? I make a habit of dipping their beaks into water a couple of times, and into the chick feed to make sure they all know where to find it and how to eat and drink. It sounds silly, but it helps. SaveAChick vitamins with electrolytes is good to add to the water instead. Check the others for pasty butt daily, and keep vents clean. She may have been a weak chick or had an internal organ problem, an infection from hatch, but hard to know. Sorry for your loss.