Enriched uranium nuclear bombs are technically much simpler to construct than plutonium bombs, once one has overcome the enormous difficulty of enriching natural uranium to weapons grade. The enriched uranium Hiroshima bomb design was so simple that the U.S. did not even bother to test it before it was dropped on Hiroshima. Such bombs rarely fizzle.
There were so many uncertainties with the design of the U.S. plutonium bomb that it was tested in Nevada (the Trinity test) before it was dropped on Nagasaki. If a test is a fizzle, it is almost certainly a test of a plutonium bomb.
The Iranians have announced that they have constructed thousands of centrifuges to enrich uranium, whereas they have as yet no operational nuclear reactors which are capable of manufacturing plutonium. Plutonium in the amounts needed for a nuclear bomb can be created only in a nuclear reactor. So the Iranians have decided to go the uranium bomb route, at least in the short run.
The North Koreans have a 50-megawatt graphite moderated nuclear reactor which is fully capable of creating plutonium of isotope 239 from the 238 uranium isotope in the reactor’s core. The North Koreans are not known to have centrifuges. Plutonium-239 is the isotope one wants for a nuclear bomb. However, if the plutonium is exposed to neutrons in the core too long, much of the plutonium-239 will become plutonium-240, an isotope which has a tendency to spontaneously fission. This means that too much plutonium-240 in a bomb will set off the explosion too early — the bomb will fizzle. Weapons-grade plutonium is thus plutonium which is 93% or more of plutonium-239.
It is possible that the North Koreans left their plutonium too long in the reactor, but more likely that they are having problems shaping their plutonium into a bomb. Plutonium has properties unlike any other element, and our designers had no end of trouble manipulating it into just the right form for a bomb.
I imagine that the North Koreans are still at least a few years away from actually having a nuke comparable to the weapons we used on Japan, to say nothing of the far more powerful thermonuclear device.