Have you wormed them lately? I know you said you sent a sample to the vet but if do a search on here you will see that its not always a 100%. I would say low production, tail down might be a good chance that worms could be heavy. Worms can also cause anemia or could weaken the chicken enough to make it susectible to other illness such as respiratory. I had some that had same symptoms and that was their problem. I had a few day before I figured it out.