Anyone know what causes a persistant cough in an infant??

Ok I have a 4 month old baby, now for the last 2 months I wanna say she has had a persistant cough and is always congested. I've taken her to the doctors several times and to the ER a few times They keep telling me its an upper respitory infection it will go away but they don't seem to be liestening to me when I tell them she's constantly like this and it's NOT going away what should I do and has anyone elses infant had this problem?? thanks