What harmed unions in the 1920s

There were more jobs and higher pay becoming available that were becoming available to more than just members of the Union.

Explanation: What harmed the unions in the 1920s was the action of Capitalism, through the signing of nothing favorable agreements for the unions and through a system of bureaucracy and exploitation imposed by the industries of the great empires, which were factors that triggered strikes and protests that ended many times in persecutions and arrests, without being able to get their labor rights recognized.