Suggest Treatment For Cough

My daughter is 6 1/2 years old and she has had this cough for about 5 years now it's not constant, but its always there. Like it's just lingering there. It's the same at night as it is in the day and it sounds like a bark or a deep down cough. She has no other symptoms except sometimes her cough occacionally gets worse. She tried different kinds of medicines but it eventually comes back.