is it a dry cough, productive cough? are you on meds for high blood pressure? statin meds actually have a side effect of a persistant dry cough.
you may have allergys, i do and it took a year for me to figure it out, could be something in your home causing it like a mold. Have the skin test done it might help
If you haven't already had a chest x-ray, you probably should have one to be sure and rule out any serious problems. Some doctors jump over that step and a chest x-ray is always a good idea with a chronic cough.