upper respitory problem in puppy

Question
I have a 4mo old boxer puppy who has had upper respitory problems since we got her.  (we got her from a pet store) She has been on three different types of antibiotics and nothing seems to help.  As soon as she gets off of them, the nasal discharge in her nose gets wore.  Her coughing is getting better, but she continues to have mucus looking stuff come out of her nose.  I've heard these things can take a while to get better, but I'm wondering why the antibiotics are not working. Any advice on what to do next.  Thanks

Answer
Hi. The bad thing about pet store puppies is that they are usually kept with other puppies that came from different breeders, untelling what they come in contact with. What types of antibiotcs has she been on and for what length of each? Has any blood work been done, if so what was done and the results?