Are you a certified organic farm? Most rely heavily on preventative diets, but it's not 100%, so they do get chickens which get parasites, and/or an illness that has to be treated with non organic medication. They either treat them, and sell them immediately as non organic, OR they cull them, OR they often set up an agreement to give them to a nearby non organic farm.
In your case, either your chickens are not getting their dietary needs met, or they most likely have parasites. Not all parasites can be seen with the naked eye, that's why fecal floats are important. Your test results will guide you in your next step. Either they need to be dewormed, or if they are parasite free, you need to change their diet, possibly to a different brand of feed. Please, don't put it off. They are already showing weight loss, and you don't want them to become more depleted, and develop other issues.
You mentioned that you changed them to organic crumble a week to a week and a half ago. What were you feeding them prior to the change?