Why do panel vents do not blow out air when you hit the AC button or panel vent button it blows out the windshield defrost vents 1989 Topaz?

most vehicles use a vacuum diaphragm to move the mode door inside the dash from the floor to the vents to the windshield. Either the vacuum diaphragm is leaking or the vacuum hose has become disconnected. In my 85 topaz the door pannel vents are used to defrost the side windows.