Third sick hen over several month span

[narly]

I have a little over 1yr americana hen that was healthy and laying until about a week ago. She first sat alone, then quit laying, ate and drank less and moved around less. Next she went to roost early and was ruffled at night and her wing drooped. She was eating some until today and she quit walking around, eating and drinking today. This is the third hen that I have had do this. The other two were Buff Orphingtons. I got all of them at the same time from the same place. There has been several months between them getting sick. The other two have died. I took one of them to the vet and he was unable to do bloodwork to tell me what was wrong and the things he gave me to try didn't work. This hen did have some mites or lice that I treated with DE. I no longer see them. I am fairly new to having chickens and would be very greatful for any info. I am afraid that I will lose all of them.