All the vent flaps are controlled by vacum. Basically, there should be a single vacum line coming off the intake manifold that runs to the passenger side and through the firewall somewhere near where the glove compartment is. On the inside of the car it runs to a reservoir in the center console. This reservoir looks like three plastic ***** glued together. From the reservoir, there is a line that runs to the control that has the 3 buttons and from there there are separate vacum lines that run to the individual flapper actuators.
You need to trace the circuit down to see where you are losing vacum. I would start at the engine. Right off the intake manifold there is a black & white check valve. Make sure that you can blow through it in only one direction. From there trace it down.