I lost a hen last night and never did figure out what was wrong with her. She was a year and a half old and never once laid an egg. When I looked inside I didn't even see the parts to make eggs. Her comb never grew and she ate very little her entire life making her very skinny. I had her from a 1 day old and she has always been "off". I have had to bring her inside many times for extra heat and even then she had little interest in food. She didn't have any parasites, was well received by the flock, loved to run around, and didn't show any signs of illness other then getting cold every now and then. Truth be told I didn't expect her to out live the brooder, I thought she was going to be a failure to thrive chick.
On a brighter note, the chicks just got to go outside for a while today. It was warmer when I put them out there then I had the brooder but quickly cooled off. My flock likes them already and all sat by the fencing talking to them. My rooster went to the top and dropped a worm in there for them but they didn't get it. They were too busy playing keep away with a twig. My head hen taught them how to dust bathe and they had a blast.
Sorry about your hen. Even the "off" ones capture our heart.