[glogla]

What does it mean for sanctions not to work?

You can see sanctions work - both in specific effects (e.g. a factory producing missiles has to stop since it can't get chips) and in some macroeconomic effects - though those are harder to disentangle from the general chaos of everything.

It is much harder to see sanctions not work, how would you even discern that?

As for Cuba, I have no idea what USA is doing there.