Many thanks to everyone who gave advice, but I am ashamed to say that I have figured the probable cause of this sickness whereby the food is not digesting, and their breastbones are just covered with skin.............I had used some old window screens to separate them away from the door so I could get to another flock of chickens and they started pecking on the screens. When I finished with the other flock the screens had large holes in them! They were EATING screen wire. No wonder their food hasn't digested! So far, I've actually lost only 2, but most of the others are still sick. I hope I don't lose all. Oh, well, a costly lesson learned! Never use screen wire where chickens have access to it!!
Oh no, I'm so sorry

I would encourage you to have a necropsy if you lose any more or if you are up to it, inspect the crop, gizzard and intestines just to make sure it's the wire that is causing the death. Here's a link to your state lab system https://www.clemson.edu/public/lph/cvdc/index.html
The metal can impact their future health. Ingesting metal objects is fairly common in chickens and it's call Hardware Disease.
http://www.the-chicken-chick.com/hardware-disease-in-backyard-chickens/