Welcome To BYC, I'm sorry that your pullets are not well.
What symptoms are you seeing?
Any photos?
Where do you live? (state?)
Are they improving with the Duramycin?
A lot depends on the respiratory disease you are dealing with. Symptoms from Mycloplasma usually respond well to antibiotic treatment. Infectious Bronchitis is a virus, so usually that will have to run it's course over several weeks, but antibiotics can help with secondary infection.
If there is a foul odor, that could be Infectious Coryza and sulfa drugs would be needed.
You can try using Tylan50 to see if that makes a marked improvement. Dosage is 0.25ml per pound of weight given orally 3 times a day for 5 days.
If there is pus in the eyes, that needs to be flushed out and removed. An eye ointment like Terramycin can be put in the eyes.
See that they are eating/drinking well. If some are reluctant to eat much, check inside their beaks just to make sure there's no mucous, plaques, sores, etc. You can also entice them with some egg, fish or meat. I would also give them a boost with poultry vitamins a couple of times a week.
Keep in mind, that most all respiratory diseases make birds carriers for life even if they recover. Here's some info
http://edis.ifas.ufl.edu/ps044