Why will air only blow out of the defroster even after changing the selector?

you have a vacuum leak.lift the hood,look next to fire wall and check the small vacuum hoses,about 1/8 and or 3/16 inch diameter.they may tee off and go to different places.check these with the engine running and a/c on.listen for hissing noise.check all vacuum hoses for leaks.when you have a vacuum leak,the blower motor will default to the defrost mode.