My gmc sierra isnt blowing air through vents?

[mxrider241@ymailecom]

I have a 2005 gmc sierra 1500 5.3l and it blows air only through the floor vents and the defroster vents on the side of the dash (between dash and door) what can this problem be? i was told to buy a new air door actuator but im not sure if this is correct