Kinda sounds like several things could be the culprit. Infectious Bronchitis, Mycoplasma Gallisepticum and Aspergillosis.
How old are they? Are they sneezing, have nasal discharge or raleing? Do they have a fluffed up appearance? What kind of bedding are you using? Have you changed the feed they were eating? Were the eggs you set dirty? Sorry for all the questions but just trying to narrow it down. Pheasants are usually pretty healthy at hatch.
At the very least I'd start them on an antibiotic, erythromycin or oxytetracycline. They both can be added to their water or in feed. Although if it's infectious bronchitis there is no medication that will cure it, only help reduce mortality rate due to secondary infections. Also raising the temperature of the brooder by 5 to 10°F may help in lowering the mortality rate, if it's IB.
IB is primarily a chicken disease but pheasants and turkeys can contract it.
Do you have chickens, also?