My understanding of the "Blend Door" is that it adjusts the temperature of the air coming into the passenger compartment. One extreme is all cooling, the other is all heating. Now most people have a middle point that they like. So the door gets used to being in that position. Dirt builds up around the door and "locks" it in place. When the door is now asked to be in someother location it might be stuck in the old location until it is bumped, tapped, encouraged by a supplemental force to the new location.

My understanding is that the blend door is located in the cab under the instrument panel. The location should be connected by duct work to both the heater core and the AC core. The bend door is located inside the duct work. There should be a metal shaft that the door pivots on. Tapping this metal shaft, or trying to move it may correct the problem temporary.

The best solution would be to open the duct work and repair the blend door. Not too easy to fix and hence not too cheap if you don't want to do it yourself, but if you do the labor yourself I don't think it should be too expensive, especially if all the problem is a build up of dirt.