[WS:] Each one is a very different case.
One case say about Germany and Italy before WW2 that fascists did what labor movement failed to do - mobilize the military and paramilitary elements. This, in conjunction with acute economic crisis and factionalism of the left that refused to form a common front with socialists and social democrats allowed them to capture the state and neutralize civil society. This is evident when we contrast these cases with Russia, where Bolsheviks managed to coop dissenting military elements to their cause.
Spain is a different case - it was a semi feudal country with strong landowning class rather than a bourgeois democracy. Again the military with strong connection to the landowning class was a critical factor in a fascist takeover.
Chile and Argentina are similar to Spain with one important difference - US imperialism that bankrolled the fascist elements. Chile was perhaps closest to bourgeois democracy and Pinochet coup would likely have not taken place had the country been left it its own devices (most conservatives in the parliament disliked Allende but were opposed to a coup). However, the US imperialism appears to be the decisive factor that pushed the country into fascism.
So the general rule holds - there has not been a communist or socialist revolution in a bourgeois democracy. There have been fascist coups, though. But fascist coup is NOT normal functioning of bourgeois democracy, contrary to the beliefs of the radical left.
Another point - the success of socialist party in a bourgeois democracy seems to depend on the strength of its material base rather than commitment of its volunteers. That is another error of the radical left - they try to mobilize volunteers instead of building the material base.
Material base in this context means organizational structures of the economy. Business has it, anti-capitalists do not. That explains the difference in political outcomes. End of story. No volunteer commitment can make up for what is lacking in the material base.
Samuel Gompers understood that very well when he tried to use capitalist institutions to advance the causes of labor. This approach eventually failed not because the idea was faulty but because of the errors in execution, such as mob infiltration, failure to include the entire working class, or reliance on Democrats instead of forming a labor party (which is the broader problem of US electoral system).
But revisiting Gompers is probably more fruitful than reliving the "glorious past" of the Bolshevik revolution. It is like feeding money into one slot machine because some was once lucky to strike a jackpot.
Wojtek