That's the problem for using Tractor Supply to do the job of a veterinarian. You are going to have problems and the birds are still going to be sick. Which vet school did the people at Tractor Supply go to? You wouldn't go to a plumber if you had pneumonia, yes?
Stop the treatment now. They should be ok for egg use in a couple weeks if only 24 hr. treatment period--most of the birds probably have not ingested any of the antibiotics (one of many problems with it).
If the birds have a respiratory infection, the first thing to do is find out why. Antibiotics are not going to help if it is a parasite or fungal disease, for example. Feedstore/Tractor Supply antibiotics are totally worthless, to the point that we do not even test their activity here in our clinic. They are immunosupressive and ans you know have egg withdrawal times or human consumption issues.
Very likely they have a combination of viral and bacterial infection. They may have respiratory parasites as well. There will be more to it than just the sneezing and cough. You are going to need local help on this, and a scientific and solid diagnosis to find safe and effective treatment.
First, catch each bird and check them over. You can examine her thoroughly, including opening the mouth and having a good look in there for mucus, redness, masses or anything else unusual. You can take her temperature gently with a rectal thermometer. Anything above 105F/40C is significant. Palpate the tummy for an egg, fluid, lumps or anything else. Check all her joints for swelling, pain, and mobility.
They need to see an avian/poultry-experienced
veterinarian ASAP for complete examination, diagnosis and appropriate treatment. Check this link
for members of AAV in your area or call your regular vet and see who they recommend; ask if they really have worked with birds a lot and especially inquire as to poultry experience.
The expense for this is going to be a lot less than inefficient, ineffective, dangerous treatments, guesswork, and loss of the flock; not to mention possible implications to human consumption of tainted eggs. Many states/governments have poultry diagnostic labs that charge very reasonable fees to test for common diseases. Because you have others and presumably use the eggs, it is important to have a solid diagnosis and treatment safe for egg consumption.
If they were my patients, I would start with complete fecal analysis and direct smear, for multiple parasites; bacterial culture and sensitivity of the feces and choana. A tracheal swab would be required to check for gapeworms. Depending on the case I might do a fungal culture. Routine blood work is necessary to rule out other issues. I would very likely order a number of DNA tests for poultry viruses as well. Generally I start them out on antibiotics as indicated by the tests (I use a lot of human antibiotics that are injectable) that are safe for minimal egg withdrawal times.
Pet/feed store medications and home remedies are harmful, ineffective, immuno-suppressive, and make them much worse and may interfere with the veterinarian's diagnosis and treatment. Do not use them.
The flock should be on a high quality pelleted diet with extra greens/pasturage. Overcrowding, cleanliness, proper water, environmental temperature, humidity, ventilation, photoperiod, and toxic exposures should be addressed.