The two factors you mentioned are indirectly relevant to Orwell's changing views on communism (specifically, Stalinism) during the Spanish Civil War. He never actually served in the International Brigades, the units which were set up by the Communist International (or Comintern), the organisation based in Moscow that sought to coordinate the activities of communists across the world. While not every volunteer was a communist, or even a socialist, the leaders of these units were almost universally communists, and a small majority (50-60%) of the volunteers were card carrying members of the Communist Party in the countries they had come from. But it was not their politics which stopped Orwell from joining them - he had appealed to the leader of the Communist Party of Great Britain, Harry Pollitt, for his support in going to Spain, but was unwilling to commit to joining the International Brigades before getting an appreciation for what was happening on the ground, and Pollitt remained suspicious of Orwell's motives and politics, and the arrangement never came to fruition. Orwell travelled to Spain instead with the aid of the smaller Independent Labour Party. Once he reached Barcelona, the local ILP representation - a British MP named John McNair - recommended that Orwell join the the militia of a smaller leftist party - the POUM. The POUM were a small, Catalonia-based revolutionary group that was opposed to Stalinist-style communism, and held that the civil war was also a revolutionary war - only by following through with the revolutionary spirit of 1936 could the struggle be successful. Orwell agreed, and went to the front with this group in early 1937.

It's worth noting that Orwell was somewhat frustrated by this experience - both because he was stuck on a quiet front (while the International Brigades were right in the middle of the crucial battle for Madrid, earning plaudits across the world) and because the POUM militia was not particularly well-led or organised. When on leave in Barcelona after several months at the front, he put in an application to transfer to the communist-led International Brigades around Madrid. However, events intervened: underlying political conflict between revolutionary factions such as the POUM and the Republican and Catalan governments (backed by the communists) boiled over in early May, as government forces sought to seize key buildings in Barcelona, such as the telephone exchange, from the control of the revolutionary militias which had held them since the start of the conflict. This was interpreted as the government seeking to re-establish centralised control in Catalonia, where a great deal of indiustry, agriculture and government had been taken over and collectivised by revolutionary gorups in the early months of the war. They faced resistance, and the result was several days of confused street fighting, in which Orwell ended up on the revolutionary side of the barricades. The POUM in particular was targeted in the aftermath as having provoked an uprising against the Republic - to aid the fascists, according to communist propaganda, which painted the POUM as secret supporters of Franco who sought to engineer divisions within the Republic to help his cause. Orwell became immensely bitter at what he saw as the ill-treatment of these Spanish revolutionaries, whose revolution was being betrayed and rolled back by the Stalinists. He withdrew his application for transfer to the International Brigades, and returned to the front, where he was wounded fairly soon afterwards. He eventually returned to Britain, and wrote Homage to Catalonia, probably the single most famous literary account of the conflict, while still immensely bitter at the betrayal of the Spanish revolution by the communists. This was very much a leftist critique of Stalinism - communism was being condemned not for wanting a revolution, but for preventing one.

There's plenty of material in the Spanish Civil War section of the FAQ that discusses Orwell's critique of the Republic and Stalinism in Spain. Some other answers, notably about the role of communists in the Spanish Republic during the war, might also be useful context. I don't want to simply repeat what they say here, but I'm happy to address follow ups if you have any.