Might help explain the gaping plot hole Lucas left by having that much of it built and taking another 18-20 years to complete it. They built the frame and had problems with the internal tech until helped by a brilliant scientist.
The Tarkin novel also specifies that they have all sorts of logistics and supply chain problems, as well as sabotage from rebels like Berch Teller's group.
Even by the time of Rebels, which is 5 years prior to ANH, they're still
trying to find enough kyber crystals for the superlaser