Never worked? Those countries are all economic basket cases. They would all have burgeoning economies if they were free, capitalist countries that did not enslave their people. It is communism that does not work. Just compare the economies of South Korea and North Korea. The megalomaniacs who run those nations prefer power over the health, wealth, and welfare of their people.
To the degree that the nuts in charge are still in power, I suppose that one could argue that sanctions do not work in that respect. But sanctions certainly keep the world much safer from their mindless aggression. There is no doubt that Iran would have its own nuclear missiles and Cuba would have Russian nuclear missiles if not for sanctions.