Why would car heater work but the defroster not clear the windows?

The heater is set on recirculate the air of the cabin. Set it on normal (from outside the car)

You have to move a button on the console to take air from outside the car or else the air will be too moist from all the humidity that accumulates in the cabin.

Also the engine temperature must be hot enough. I usually run my car at least 5 minutes driving it downtown before it is hot, during cold winter days.