In addition civil war, kulaks burning food, imperfect management science (Taylorism which had problems with adjusting controlling to the reality on the ground), natural disasters and bad climate for crops. For example even when harvest was going on in plenty of areas the weather was too damp to gather crops at the ideal point in time which greatly diminishes your harvest. The same process could be seen this year in Europe.

The UK did embargo the Soviet Union till Lenin's NEP and similar things did happen regularly, this means that international finance and industrial capital would've often sought other countries in which they didn't have to fear such things, too. This means that the Soviet Union had to try to generate capital from other sources and those are the the surplus of the working class or the savings of people (vs. consumption).