I realize it's been a while, but I thought I should update you all anyway. I did as you guys suggested and got her some chick vitamins. She was doing better for a while until one morning I went to check on her and she was gone. I had her in the bathroom, which we had been renovating some time before. Apparently there was still a hole in the floorboards we hadn't seen and we believe an animal may have taken her. My poor sweetheart.
Good news is the others are all grown and healthy and outside with the others. So that's something.