HVAC Duct

Probably a vacuum problem, start at the control head and check for vacuum there, if there is no vacuum present then you need to follow the supply line back and look for a break in it. If you have vacuum at the head then the problem is between the actuator and the head, again start chasing the line looking for a break.
By the way, the supply line runs into the car from the pass side of the engine compartment, make sure all is good on that end too if no vacuum at head.
Good luck.