1991 BMW 535i Not blowing thru vents

I have this BMW the air condition is not blowing thru the vents. However you can here it blowing. I took the dash out as far as i could but did not see any blockage. It sounds like the defrost is on but not sure. No matter how i manuever the switches to turn the front vents on or off it keeps the defrost on. If I turn the air down it does not work you have to keep it on the highest level possible to allow it to work. Thats the best I can explain hopefully you can help