5.9 Magnum, also 2wd if that helps

 

You may not have enough vacuum, Some of the actuators have springs in them and the vac needs to be strong enough to pull against the spring tension before the vent will move. Have a look at the vac line going to the cruise actuator under the battery. It's notorious for cracking and leaking.

 

Checked that and its not leaking, However my heat seems to work slightly better, But we will see how it is in winter