[beautimus]

As one of the guys above stated, it can also be the water side. if you have plenty of air flow and the air is not hot, then the heater core may be plugged up. i have ran into this myself. when the heater is turned off, the heater control valve shuts down the flow of water through the heater core to keep from roasting you during warm weather. so the stagnated warm water begins to let the disolved minerals drop out and deposit on the interior surfaces of the core. given enough time this can totally block the heater core.
another factor is the use of "bars leaks" this is a product that is sold in a lot of auto parts stores that can effectively plug pinholes in radiators. this stuff will get you home but if you use it, you need to flush the cooling system and change the coolant afterwords. i have repaired this issue on several cars by simply using radiator flush and new coolant.
the next culprit is the thermostat. if it sticks in the open position, the water may never get hot enough to heat the core properly. in other words the water does not stay in the engine block long enough to capture all the heat that it can, before it goes to the radiator and gets cooled off again.
Good luck! Tony