Before I get into specific reasons as to why "communism never works", I would like to clarify a few things.
-Communism =/= Totalitarianism
-Communism =/= Soviet Union
-Communism =/= Stalin
There are two reasons in particular that explain why communism has "never worked" which I have deduced and would like to bring up.
1) Firstly, communism has only existed as an ideology since the 19th century, and has only been attempted a handful of times since then. Capitalism, or at least some form of free-market system has existed for thousands of years in hundreds of different civilizations like:
---Numerous city states during the Sumerian Renaissance (2111 BCE - 2004 BCE),
---Intrastate trade within the Achaemanid Empire and later the Hellinistic Kingdoms (550 BCE - 63 BCE)
---All across the Mediterranean region in Greek/Phoenician city states and later within the Roman Republic (500 BCE - 500 CE)
---The Silk Road in antiquity and Pax Mongolica (100 BCE - 1400 CE)
---Medieval Islamic Caliphates and Post-Medieval Europe (660 CE - 1650 CE)
---Possibly numerous other, unrecorded times in historical regions such as Mesoamerica, North America, the Andes, Sub-Saharan Africa, Australia, and Polynesia.
The modern incarnation of capitalism is merely the most recent elaboration on a system which has been in place for centuries. There have been numerous times in history where the system of capitalism has led to the downfall of civilizations, such as governments that fell to economic collapse in Indonesia and Indochina during the Medieval Period. Some of these civilizations were ultimately destroyed because of their free market economic system, and yet the free market is still widely viewed as a successful system. So, communism itself has not been given enough opportunity to become successful, or have the same time for it's infrastructure to develop into an effective system.
2) Widespread communism was spawned in a time of conflict. The very idea of communism was under attack by ideological disagreement from the first world. But not only that, each individual nation which adopted communism was either at war, or undergoing a harsh time economically, more often than not, both.
---The Russian Revolution occurred while Russia was at war with the Imperialist Germans, and the contemporary Russian Empire had suffered more than 2 million military deaths and over 3 million civilian deaths due to military action and famine (this amounted to approximately 2% of the population at the time). Mass starvation was already a rampant problem when the Bolsheviks took over and with the rise of the White Army that same year, a further million+ Russians, so in the course of ten years, from 1914 to 1924, Russia had lost millions to both war and famine. A country which had lost millions, and continued to lose millions, and was constantly under attack needed strong leadership. Even capitalist countries have only endured war through strong military leaders (Churchill, etc.), and with the premature death of Lenin, there was a call for strong leadership. The call was answered, fortunately or unfortunately is up for debate, by Joseph Stalin. A few years later, the Soviet-Japanese Wars in the 1930s and 1940s, and of course, World War 2, resulted in a further nearly 24 million civilian and military casualties. My purpose in this, is to point out that the Soviet Union was born into a time when constant death and war was commonplace, a theme which will be found in these other examples.
---China became communist during the Second Sino-Japanese war which was one of the bloodiest wars, in terms of total casualties, in all of recorded history. The Imperialist Japanese tortured China for eight years and killed an estimated 2 million Chinese. Also notable is that, while the Chinese government was struggling to remain intact during this time, it was divided between two major parties: the nationalists, and the communists. So, even after the Japanese surrendered to America in 1945, those two parties continued to fight among each other, resulting in further conflict during the onset of the Cold War.
---In the aftermath of the Pacific War, anti-European sentiments in French Indochina had grown, and when they were denied independence, they did what almost every group did when denied independence from overseas rulers: they fought for independence. And the French, who were only just recovering from the damage done by the Nazis, called for foreign aid, which was not immediately answered. But when the revolutionary leader, Ho-Chi-Minh, adopted communism as the ideology for the fledgling nations emerging in Indochina, America sprang into action to prevent the further spread of communism. This ultimately resulted in some of the longest, most expensive and taxing wars for both sides, of the twentieth century as America tried its best to suppress communism in Vietnam, Laos, and Cambodia.
---Korea had been under imperialist occupation by Japan for over thirty years, and when the Allies advanced into Japanese-occupied Korea, the land was divided according to which ally "liberated" the region. The North went to the Soviet Union, and the South went to the United States. The already devastated Koreans were further divided by the ideological differences of their benefactor superpowers.
---Cuba was the only officially communist country in the entire western hemisphere, and when the Cuban Revolution occurred, the United States placed a trade embargo on Cuba, which essentially ended any chance Cuba may have of economic growth, and with the US already on a paranoid defensive because of the Cold War, Cuba had to devote its entire focus to military protection, which led to the government vying for absolute power, which was not a difficult accomplishment in the already fractured Cuba.
No country under the circumstances I've listed above can flourish, regardless of their economic system. There are capitalist countries under similar conditions all around the world in Africa, Latin America, and Asia, but people don't consider this grounds for the invalidation of capitalism itself, so considering this grounds for the invalidation of communism is ignoring a lot of facts.
I hope that I've shed some light on this subject for communists and anti-communists alike, and hopefully opponents of Marxism who have used this argument before can understand my point of view on the issue, and maybe have a bit more open of a mind.