I’m not very knowledgeable with chickens but one possibility if everything else can be ruled out could be kidney failure. One of my geese has kidney problems and her only symptom is constant resting. She sits around most of the day. She eats and drinks fine, is in good spirits, but she still sits around a lot.
If it’s a kidney problem the only treatment is lowering the protein intake, limiting stress, and giving tart cherry extract and apple cider vinegar to help the body rid itself of toxins. Others with birds with this conditions have gotten a few more good years with their birds so it isn’t necessarily the end, though it is progressive.
