Communism doesn't work simply because the necessary steps to introduce it cannot be feasibly introduced.
1) You need a charismatic ruler willing to take charge so that the socialist state of change can be implemented. You then need this ruler to go away so that the communist republic can be established from the evolution of the socialist society.
So you need a disposable king, in other words, because the sort of person necessary to form the revolution in the first place is -exactly- the sort of person you do not want running the whole works.
2) You need a self-sufficient economy, where the people do not want for their necessities, a land of plenty. The moment you have shortages, you have a starving populous that cannot attain more for themselves through the very system designed to spread the wealth, so to speak. A society is exactly three meals away from anarchy. The populous has already proven to be able to overthrow one government; they can just as easily do away with yours if their kids aren't fed.
3) There is no impetus for improvement, no competition that bolsters change. With communism, there's no reason to improve upon products, because that's an investment of resources that could go, instead, to -making- those products. And because distribution of those products is guaranteed, and because your compensation and your own personal wealth are not challenged by outside forces. If you're in the mousetrap business in a non-communist country, you need to make better mousetraps, or someone else will. And if they do, for a better price, you starve. Thusly, the general quality of life improves over time. In a communist country, building a better mousetrap means more resources spent, but there's no increase in profit from that, so there's no point for you to do it. There's no decrease in profit from not doing that. You still take home the same amount of resources to feed your family either way. So why improve?
4) Because of the need of self-sufficiency, and the lack of improvement, your ability to trade on the world-wide scale is signifigantly curtailed. No country will look to your country for quality products, unless you're the only supplier in the world. This means that the only ways your country improves it quality of living in the global competition is militarily. And without civilian improvements in science and technology, this is much harder to attain.
5) Eventually, either your country will collapse from economic ruin (no competition), from military takeover (your military cannot stand against a world constantly improving forever), from starvation (the people don't get their basic needs), or by absolute necessity, some system of competition and merit forms in the underbelly of your society, creating the very social structure that communism is supposed to depose.
In other words, communism's survival is only assured when it ceases to be communist.
Communism only works in theory because the theory excludes 'The rest of the world' in the equation.
It works for small groups, however. Households can be communist -easily-, as can larger town-sized communites. But a country is too large and anonymous for it to work out for long.