Economic sanctions are killing North Korea and they have run out of enriched Uranium, Plutonium, Tritium and other critical bomb elements due to economic sanctions cutting off supplies of raw materials or, more likely, because President Trump has shut down supplies of raw materials and probably also weapons grade or near weapons grade fissionable from suppliers like Russia, Pakistan and China.
No way could North Korea have been keeping up the pace of their nuclear weapons and missile programs without massive outside support in all forms - financial, technical and nuclear weapons specific materials
Is it possible that the threat of tariffs on Chinese products is also causing some movement here?