I have just finished typing a response to a very similar question in another thread. I will copy and paste what I wrote in there, hopefully it will help.
"There is a possibility it may be poisoning from mould - I had this issue a few months back. You need to check every square inch of your coop for mould spores. When I did this I found a very tiny amount under the waterer, this was enough to bring 6 full grown brahmas down - it really is that dangerous. Once i discovered what it was I had to clean and disinfect the coop within an inch of it's life. I was told by an extremely knowledeable person to give them selenium and vitamin E capsules. Pierce the capsule and dispense the contents onto a piece of bread or similar. One capsule per chicken per day. I think the selenium dosage is 50mg but I will check that and get back to you. Activated charcoal tablets are also useful - crush a couple and mix in with some mash or porridge.
All but one of my birds recovered in the end - I had to cull one sadly. About 3 months on some are still not laying however so be prepared for full recovery to take some time. Good luck."