one thought you are letting the car get up to temperature arent you, if your just setting the car to a temperature then the engine needs to get up to temp before it can blast hot air from any vent, its as if it knows what temp it can output and steps up / down the speed to match its output, so if the engine is cold it wont do much (unless you tell it max then it will just blast what ever it can whether its up to temp or not)