I could be wrong, but it sounds more like the vacuum relay and not the actuator or vacuum canister. If your headlights are quick to pop-up it sounds like the proper vacuum is there for operation.
It sounds like the controlling vacuum that is causing the problem. The tiny hose on the vacuum relays are the control line. (If disconnected the headlights should open up.) Retrace the tiny hose and see if it is OK. Mine was squashed inside the hose holder on the fender, and prevented the vacuum from operating.
The control vacuum line originates from a "T" fitting near your brake booster. It goes into the driving compartment and to your headlight switch. It then goes to your headlight override switch (under dash between speedo gauge and tach). After that, it goes to the vacuum relays at the front of the car.
Check it out and see if those hoses are still OK. (not squished). I can post a diagram, but I have to draw it up first.
-Vic