The Nissan Path

I have a 97 Pathfinder with a heater that works great on Defrost.Once I switch it towards floor heat I immediately get what seems to be direct outside airThe mode selector switch works great and changes through every mode.This problem happens the moment the selector is moved one notch towards floor..Can someone tell me what the problem might be? I'm trying to avoid tearing into the duct work without any direction.

thanks