Due to shortages many basic products were rationed and you simply couldn't buy them with cash. Having more money didn't make a difference unless you had the right rationing stamps (or connections to a high-ranking party official). This lead to a whole secondary market of people trading stamps with each other (e.g. cigarette stamps for meat stamps). When it comes to accumulation, periods of hyper-inflation and lack of investment opportunities made this much harder. Moreover, currency exchange rate was controlled by the government, which led to illegal underground currency trading.
This is based on experiences from Poland, but I'm sure many other communist countries had similar issues.