Took 'em five test shots before it worked right. The first four Nork tests had yields in the one kiloton area. That's a fizzle in most people's books. Where as the United States was able to produce TWO functional nuclear weapons in 1945 with only one test shot. The Little Boy gun type uranium bomb was so well understood that it was sent into action without a test, and produced a 20 kiloton yield that devastated Hiroshima. Fat Man, the far more tricky implosion type plutonium bomb, was tested once in Nevada before being dropped on Nagasaki. In action, it worked properly, with a 20 kiloton yield, and vaporized Nagasaki.
Getting a fission bomb to explode is tricky. You have to assemble a critical mass of fissionables, either by gun style assembly or implosion, and hold it together long enough (nanoseconds) for the neutrons to fission the fissionables. If the energy released in the first few nanoseconds blows the bomb to bits, you don't get a 20 kiloton yield, you get a fizzle.
We let the Norks run off five nuclear tests, and finally they got it together, achieved city smashing yield, and we did nothing to stop them.