My Granddaughter Is Almost 7. She Had The Flu And

My granddaughter is almost 7. She had the flu and now has a horrible cough that nothing we try is calming down. We have tried vicks, cough drops, cough syrup, steam, a humidifier, and breathing treatment with prescription meds. The doctor says she may have asthma and is susceptible to pneumonia. What else can we try to sooth the cough?