As Mad-as says... I've pulled a number of the heater boxes apart and freed up the flaps in them. I usually ream the holes out in the heater box itself to give a bit more clearance for the flaps to move in, and also I've found in a number of them that the grease that was used originally had dried up and also instead of keeping things moving nicely actually helps bind things up.
I use silicone grease when putting them back together, rather than the white lithium grease (as it seems to get less greasy and more solid with age).
Also worth checking the alignment of the gears on the main grey spindle, as if they're out a bit then one set of the 3 flaps will get to the end of motion before the others and stall the motor out.
I've also found on a few heater boxes that the drive shaft from one of the sets of flaps has slid out over the years, so one set of the distribution flaps isn't driven properly (if at all).
Incidentally - the last few heater box out/heater core/blend motor swaps I've done for other people have been on later 2000/2001 vehicles and the heater boxes haven't needed any work and seemed a LOT freer than earlier ones. So I wonder if there was a slight modification in the production on them, as the later ones did have a different part number....
I believe you should be able to move ALL of the flaps (including the distribution flaps from one end to the other) with your little finger. The motors have a fair bit of torque in them due to the gearing, but obviously the freer the flaps, the easier life the motors have. I've managed to get some of the earlier heater boxes to be pretty freed up - but a couple of the later 2000/2001 versions that I've pulled out have been really free and easy to move - which makes me think they must have adjusted the tolerances in later production.