I am new to chickens & BYC but something very similar happened to our new flock!!
After we built our new coop we purchased the majority of our chickens from a farm out in Kearney MO but we did get 3 chicks from a little urban coop that a friend had. Those 3 chicks seemed very healthy and same went for the more adult chickens we got from the farmer. After about 2 weeks one of the cinnamon's we purchased from the farmer started having a clear runny discharge from her nose (I didn't think to smell it) we were told from friends it happens sometimes and give them some vinegar or if it gets worse tetracycline. Well we noticed our first sick cinnamon getting very ill and others started to display the same symptoms so we went for the medicine and for over a week mixed it in with their water. All the others seemed to make a full recovery and no more goo from the nose from any but that first sick hen is still so lethargic. It is odd. She seems to have energy in the morning to go out and peck around for a bit and eat but later in the day she looks so exhausted she can't even open her eyes. One day I found her out in the yard at night just laying there after all the others had put themselves to bed! I haven't given up on her yet but was also looking for some advise in this area. They were all laying really well the first couple of weeks and then went on a hiatus.
Now we are getting about an egg or 2 a day from 10 hens (seems low but but 5 of them are not quiet old enough to lay yet) Still seems low to me though...Any advise would be greatly appreciated!