Talk to General Physician on Dry Cough

AoA, My mother is having dry cough since last 7-8 mnths , got tested several times, all test are clear but no improvement in cough, tried a number of different medicine but of no use , only tab betnisol and syrup ventoline temporarily improve condition but as soon these medicines are stopped , coughing starts after 2-3 days. She is also taking concor 2.5 mg due to instable BP and pelton c (occasionally) for stomach disorders. Plz assist .