Sounds like a vacuum problem. You will need to trouble shoot the lines, actuator valves, and the integrity of the control head valve.
My best suggestion is to pull a vacuum on each line while the key is on and the fan is running. The easiest way is to actually suck on the line with your mouth. It would be better if you could use a vacuum pump.
You can do this both ways. One, by pulling a vacuum direct on each actuator in the air system. Second, by pulling the main vacuum on the control head, running it through the different zones and checking to see if you have vacuum coming out of the right ports.
I am sure there are some better suggestions out there, but this should get you going if nobody comes up with anything better.