Define the difference between an economy which 'works' and one which doesn't.
As intended. In this case, the Soviet planned economy was intended to achieve 100% employment, eliminate economic shocks, and increase output to a certain level in each industry as decided by the State at the beginning of each five-year plan. While everyone in the USSR was technically employed, misappropriation of funds caused drastic deficits in productivity in certain areas. Since managers were punished for failing to meet quotas, many instead lied about reaching them and spoofed their stocks for audits. This information became part of the basis for the next plan. Any system that works on faulty information, in anything, is going to be faulty. Garbage in, garbage out. Even if it's faulty, it can continue to function at a reduced capacity in this case, all the while causing more inefficiency and waste.
It's well-established that economies grow approximately exponentially. Errors in planning, if not corrected for, will likely grow exponentially as well. This causes the long-term deterioration of the system.
Additionally, planned economies cannot act on unanticipated events such as the introduction of new technologies and shifts in the global market. This feeds into the perfect information problem and furthermore introduces a delayed response, which fundamentally disadvantages it in comparison to a market economy.
Planned economies have enjoyed limited success in just about every country they have been tried. Most of them switched to market economies and did much better after the transition. Command economies today include Cuba, North Korea, Myanmar, Saudi Arabia and Belarus; the only one that is doing well economically is the one with massive amounts of oil and a market to supplement it.