On the one hand, Marx anticipated communism rising up through society organically from the proletariat, whereas in most communist countries it was instead imposed in top-down fashion by a so-called "vanguard party" (see https://en.wikipedia.org/wiki/Vanguardism).
Vanguardism was a feature of the Leninist approach to communism, and Lenin had not-terrible reasons for preferring it -- namely that by the 20th century, people had been waiting more than 50 years for communism to bubble up organically the way Marx had predicted it would way back in the 1840s, and it never seemed to, you know, happen. Lenin argued (in What Is To Be Done?: https://en.wikipedia.org/wiki/What_Is_to_Be_Done%3F) that Marx was wrong in this regard, that the only way the working masses would ever embrace communism was if a narrow, elite activist class first educated and agitated them.
Vanguardism worked, in the sense that it resulted in the creation of formally communist governments, which waiting for Marx's prediction to come true had never (and still never has) done. But it also tainted those governments (which were of course staffed top to bottom with members of the vanguard party) with a strong sense of their people as cattle who needed to be herded by their betters, which then made it easier to abandon democracy, shutter the free press, and murder lots of people whenever they got in the way of the next five-year plan.
On the other hand, complaints by communists that existing communist governments weren't practicing Real Communism™ always had a whiff of no-true-Scotsmanism about them. Most countries with these governments were pretty miserable places to live, so they weren't exactly great advertisements to people who lived elsewhere for the idea that they should embrace communism themselves. So people who advocated for communism had to somehow find a way to assure people that the communism they were preaching wasn't the same thing as the system down the road that couldn't feed anyone and was riddled with secret police. Accusing the folks down the road of deviationism (https://en.wikipedia.org/wiki/Deviationism) was a useful way to do that.