Heater blows but doesn't always come out vent/floors. fogs windows. Defrost is ok

[Z Long Bed]

I did try to search but didn't see anything.
So it seems as though when I switch the controls from defrost to vent, sometimes the fan doesn't divert to the vent position. The blower sound gets louder, but no air comes out.
The defrost seems to work ok.
Sometimes it diverts ok and the heater works great. This has been going on for a least a year.
Thanks for any help.

 

[scotttick]

Make sure the mode door actuator is working properly. It is near the floor,just right of the gas pedal. It diverts the air to defrost, vent and floor.
https://www.youtube.com/watch?v=Ow0OOfIFtlU