Baby chicks are very fragile and some do die. It is a sad but true situation.
They need to be protected from drafts and kept warm (90-95 degrees the first week), reduce the temps 5 degrees each week, Keep an eye on the chicks and their behavior will tell you if they are too hot or too cold. At the edges of the brooder - too hot (trying to get away from the heat); huddled under the lamp in a pile - too cold.
Baby chicks need access to fresh clean water daily. I add Quik Chik to my new chicks water for the first couple weeks. Its a vitamin mineral supplement.
Chick Starter should be available as much as they want to eat.
I had my chickc vaccinated against Marecks disease and for coccidiosis. I also gave them GroGelPlus upon arrival. This is also a vitamin mineral supplement.
Don't handle them overly. They are cute and cuddly, but resist the urge to pick them up and handle them alot. They can pick up viruses from people and we can pick up viruses from them as well. Sounds like you are already doing most of this, don't take it personal. Even the best people lose baby chicks now and again.
Hope these things help.