Help with sick chicken, one died already

I have a flock of 15 chickens. About 2 months ago, one chicken looked sick, then got better. About 1 month ago, a different chicken got sick, continued to get worse and eventually died. Today, a different chicken is showing similar signs.
Here are the symptoms

puffed out appearance
stands away from other chickens
keeps eyes closed frequently
no desire to eat, scratch or look for food
will perk up and try to get away if you want to catch her, but will settle quickly in the same droopy position
back end looks kind of dirty...not sure if that's a symptom or not, some other chickens have that and they don't look/act sick

as it gets closer to the end the symtoms are
more upright stance with tips of wings touching the ground
head buried in feathers, so they appear to have no neck and then eventually head falling forward with beak touching ground
if you try to move the chicken they don't resist at all.
easily unbalanced, will nearly fall over if moved and will use wing to try and balance.

My chickens free range outside and are housed in a metal pole shed in a stall. Since the second chicken I've been adding Vital (an electrolyte) to their water about once a week. They are all nearly one year old. They were raised on non medicated starter, no vaccinations. No deaths/illnesses prior to the onset of this. I have horses, goats, geese, cats and dogs. Chickens are of mixed breed, this new sick chicken is a silver laced wyandotte.

I did try some antibiotic from the feed mill on the second chicken, but it was towards the end. She may or may not have ingested any and/or it was too late anyway.

Any idea what is going on and how to cure?

 

I called the hatchery and they suggested giving an antibiotic to all the chickens as they probably have some sort of sickness/illness. They also said stop feeding any kind of scratch or corn and get the chickens as warm as possible and make sure they have a complete feed with 18% protein.