What area do you live in? Eyeworm is rare, and more in tropical areas of this country and the world. Much more common, are respiratory diseases, such as MG and coryza. These are viral or bacterial, and can have symptoms like a cold, but are chronic, and may make carriers of your flock for life.
I am having the same thing going on with my chickens. Im in FL. I cant figure it out i tried tylan 50 it worked but it came back. I just called my state vet today had to leave a message.
If they have cold symptoms, then it is probably a respiratory disease, which can be caused by virus, bacteria or mycoplasma, and by fungus. You might want to give them some fluids and help itypil