Not all countries joined the League.
Although the idea for the League of Nations had come from Woodrow Wilson, there was a change of government in the United States before the signing of the treaty and the new Republican government refused to join. As a punishment for having started World War One, Germany was not allowed to join and Russia was also excluded due to a growing fear of Communism. Other countries decided not to join and some joined but later left.
The League had no power.
The main weapon of the League was to ask member countries to stop trading with an aggressive country. However, this did not work because countries could still trade with non-member countries. When the world was hit by depression in the late 1920s countries were reluctant to lose trading partners to other non-member countries.
The League had no army.
Soldiers were to be supplied by member countries. However, countries were reluctant to get involved and risk provoking an aggressive country into taking direct action against them and failed to provide troops.
Unable to act quickly.
The Council of the League of Nations only met four times a year and decisions had to be agreed by all nations. When countries called for the League to intervene, the League had to set up an emergency meeting, hold discussions and gain the agreement of all members. This process meant that the League could not act quickly to stop an act of aggression.