Hot air does not come out on 1993 Nissan Altima

Why when i turn on the hot air to defrost the windshield the air does not come out from that vent but it keeps coming out from other vents , the heat works but does not defrost the windshield please send answer to

Asked by for the 1993 Nissan Altima
This is caused by a vacuum leak in a detached or damaged vacuum hose under the dash or hood.
1 more answer
When you select to blow air on the windshield by selecting the specific windshield vent, it will blow cold air automatically, unless the A/C is turned off!
This is because, during cold/rainy weather conditions, the cold (dry A/C cooled) air dries the windshield more effectively than by blowing hot air.