Poor things. I will tag
@Wyorp Rock, and
@Eggcessive for more help.
Have you added any birds to you're flock recently or visited other fowl recently? Have you checked in their mouth for any lesions? How do their crops feel? What's the approximate temperature in the coop?
Respiratory diseases have become a serious problem with chickens. There are several that can affect chickens, but, in most cases, its either Coryza, (MG) mycoplasma gallisectum, or Infectious bronchitis (IB). If you have a vet that does poultry in your area, you should be able to get antibiotics from there, but if that's not an option there are several that can be purchased online such as Tylan power, Duramycin powder, and some other fluoroquinolones type antibiotics. If it is respiratory disease-related, these will work better then the amoxicillin.
It should be understood that with most respiratory diseases, antibiotics only relieve the problem, birds are usually carriers for life. During times of stress, symptoms may reappear.
Since, dust and ammonia can have negative effects on their respiratory system, keeping the coop clean, and ensuring there is adequate ventilation may help the problem.