What was One big problem for marxists following the installation of communist regimes was?

depends on which country. in Russia it was probably the NEP and starting the collective farming process as well as rapid industrialisation. in Cuba it was getting out the corruption and foreign influence on the island. in North Korea it was rebuilding the entire country from bombed out earth. no two are alike in that they all started based on material conditions present in their country at the time. Russian socialism was different than Chinese is different than north Korean. they all adapt marxist principles to fit their own culture so they all have different problems after their respective revolutions.