Heater barely blowing out of vents

[mechman600]

I am wondering if there is a common solution to a common problem with my heater.

Even with the fan on HIGH, barely any air blows out of the vents.
-All positions work
-No difference whether it is on FRESH or RECIRC
-The fan itself blows like crazy with the fan box to heater box pipe removed
-Fan box to heater box pipe is not plugged
-I can see the HOT/COLD flap moving through the inlet hole to the heater box
-No difference whether it is on HOT or COLD, so the problem isn't likely a plugged heater core

Basically, the blower fan blows a ton of air, but there is a restriction somewhere in the heater box.
I am out of ideas. Is there some sort of filter [that could be plugged] in the heater box that I am unaware of?

Thanks in advance!

[crazycanadian]

Window cowl area... good place to have the intake area plugged up... Heater core's them selves get plugged up with dirt over time... door flaps that look like they are moving, but still causing restrictions..

[Daninski]

Make sure when you select vent the system is moving all the way to vent not just part of the way..