are you talking about the movement that lasted for 3 years? the movement that ultimately gave way to francisco franco? i'm not really well-versed with the history of anarchy in societies, but it seems like they usually don't last very long.
Yes, the forces of Francisco Franco eventually won the Spanish civil war, leading to the dictatorship. But my point is, it didn't fail because of a lack of internal organization. It, as well as other similar movements throughout Europe (in Greece, Italy, etc.) in the period immediately after the Second World War, failed due to external attacks. In Spain mainly because of the Fascists and Communists, in other parts of Europe they were "liberated" (read "slaughtered") by the Allied forces.