WWI complicated things, eg Ukraine was given up as part of the deal to get the Leninists out of the war. WWII probably didn't help, at least in some ways. It did help with unity but a lot of Soviets died, either directly in combat or as part of the Nazi purge (Slavs/Russians weren't seen as much better than Jews by the Nazis).

There were several factions in a civil war (Leninists, other communists, anarchists, capitalists) and all the alliances and betrayals that goes along with that. Plus outside capitalists didn't want to see socialism or communism succeed and played interference, though I don't think that ramped up that much until after WWII, where wars were fought to stop the spread.

Stalin had a bias for "communist-like" scientific theories, and there was one about how to grow food that came up just as a lot of farm land was being reallocated from long time landowners to people who didn't have much experience farming. These theories weren't very successful but those involved had a personal vestment in making others think it was working rather than failing (because gulag vs living nicely). So reports said there was tons of food being produced while there wasn't, leading to famine, which makes it harder to bring up a brand new economy.

China also fell for those false reports and adopted the fake science themselves, which contributed to their own famine.

I don't recall a lot of the details of that fake science, but a couple of the ideas I do remember: fit as many seeds/plants as possible into any given area because the plants will work together to strengthen their comrades. If you store seeds in the cold, it will make them more cold resistant, allowing crops that wouldn't normally grow in some areas to thrive there.

Just those two things plus the land redistribution resulted in people who didn't know how to farm trying to grow more crops than the land would support, sometimes picking a crop that wouldn't even grow in that area over less desirable ones that would.

And of course it didn't help that the Kremlin still feasted while (false) reports said there were tons of crops being produced, which were also taxed as if the production numbers were valid, so those farmers were also starving, which killed the public support momentum and they had to deal with Stalin's brutality on top of that.