Blend door actuator?

When I start my vehicle to let it warm up the heater blows hot air and it blows it like it should on high, but when I drive down the highway for about twenty to thirty miles and go to turn the fan up, it barely pushes any air out. I can hear it blowing under the passenger dashboard, but there is barely any air coming out of the vents. It does not have a problem heating. It only happens when I am driving down the highway for a ways. I have cleaned the in cab filter but have not tried anything else. When I pull the filter out and turn the fan up it blows air out of the filter a lot good, but when I put it back together it still does not push any air out of the vents, does not work on floor, front vents, defrost, and also does not work on recirculate.