Something I don't understand...

Having nuclear weapons always goes hand in hand with having the missiles to deliver them, and there was talk in the media a few weeks ago about North Korea having received theirs from a Ukrainian factory:

North Korea’s success in testing an intercontinental ballistic missile that appears able to reach the United States was made possible by black-market purchases of powerful rocket engines probably from a Ukrainian factory with historical ties to Russia’s missile program, according to an expert analysis being published Monday and classified assessments by American intelligence agencies.

The studies may solve the mystery of how North Korea began succeeding so suddenly after a string of fiery missile failures, some of which may have been caused by American sabotage of its supply chains and cyberattacks on its launches. After those failures, the North changed designs and suppliers in the past two years, according to a new study by Michael Elleman, a missile expert at the International Institute for Strategic Studies.

You certainly don't get the impression that a country barely able to feed itself is capable of making anything but the most basic missile.

So - why is it so hard to stop the supply of missiles to North Korea?   Did they receive a very large batch a few years ago that they are just working their way through now?    Surely, you would think, the Ukraine and even Putin would think it a good idea to work hard on the matter of black market supply of missiles and missile components?