Treat it like an electrical ckt., start at the light switch and make sure you have vacuum feeding it. Move down the line from there until you find the problem. A vacuum guage helps but isn't necessary.
Spend $25 bucks at Harbor Freight and get a Mitivac system. This is a hand powered vacuum pump with a vacuum gauge. It is great for bench testing your actuators and for checking out your hoses and check valves.
It also comes with everything you need to vacuum bleed your breaks. Great tool!
I spent several days troubleshooting my system. I thought it was the relay that sits on top of the tank because it was leaking at the bottom. I replaced it and they still didnt work. It turned out to be the actuator had a leak. I dont know if this will mean anything, but the one that leaked was real easy to operate by hand. The good one had some drag on it and you had to use some force to open or close manually. When I got the new one, it worked like the other good one. You may want to check them if you cant find any other problems. Mine work fine since I replaced the bad one. Good luck!!