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Is your outflow valve closing all the way? Can you pressurize the cabin on the ground (with external equipment)? If the controller is not closing the valve, you could have the tightest cabin in the world, but no pressurization. There is also an emergency outflow valve that could be stuck open...
How is the MP on your engines, at surface and at altitude? You should be able to get 37" (35" on some intercooled airplanes) on takeoff. At 20K, you should be able to pull 27" at 2450-2500 RPM (but not at lower RPM, regardless of what book says). If your turbos are not boosting, your pressurization will be weak as well...
Have you checked your dump valves (two handles on the right of center console, below instrument panel)?
The controllers themselves are usually very reliable, but I suppose you could have a problem there...
I am sure others more knowledgeable than me will have more to add to this...
Kevin
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