There are quite a few respiratory diseases chickens can come down with. Some can arrive from fairly long distances away, from other chicken flocks, carried by wild birds, etc. etc. Infectious bronchitis is common, so is laryngotracheitis. I don't even know if I spelled that one right but it causes them to have mucus plugs in their throats, making breathing difficult.
If you Google poultry respiratory diseases you can get an idea of whats out there and try to match up the symptoms your birds are having. Although many of these diseases show such similar symptoms they can be very hard to diagnose without having labwork done, or a necropsy on any that die of it.
Chickens generally need antibiotics when they come down with a respiratory disease because they so easily, and quickly, morph into pneumonia and secondary bacterial infections. Added warmth, especially at night, while they are sick also helps a lot.
I've used Vet Rx, rubbed on their beaks, to help open breathing passages. You can also research Oxine AH. I swear by the stuff and I use it to fog coops with the birds inside, let them breath the mist. Just make sure you use it as is, do NOT use the activator as it will then not be safe to use around the birds.
Good luck, hope this helps. It's so hard sometimes trying to figure out the problem and how to treat.