If it is not an environmental issue, I think that you could be dealing with infectious bronchitis. Sneezing is the biggest symptom, and when my chickens had it, I found wrinkled egg shells once or twice. IB virus is the most common respiratory disease, and it has to run it’s course over about a month. Most chickens survive it, but can have reproductive problems in the future. They remain carriers for 5 months up to one year, and it tends to spread through the flock over weeks to months. Make sure that bedding is clean and dry, prevent water spills, and maintain good overhead ventilation. If you do see any bubbles or foam in an eye, or more serious symptoms, Tylosin can help. But antibiotics will not help if it is a virus.