Reasons why the League failed

Weak Powers- countries were unwilling to send their armies abroad to fight and enforce the League's decisions, while the League did not have their own Army

Absent Members- the world's leading economic power, the USA had become isolationist and non-members continued trading despite sanctions or bans. Also, the USA had not joined the League because the republican US congress had refused to join the League of Nations as they were against the Treaty and the LON.

Slow decision-making- it took one year for the report on the Manchurian crisis to be written and while the League discussed the Abyssinian crisis (10 months), Mussolini planned and executed a full-scale invasion.

Depression- countries were unwilling to impose economic sanctions, fearing it may damage their economies. 

Unfair Treaty of Versailles- many countries now believed the Treaty of Versailles was unfair and allowed the German Navy to expand under the Anglo-German Naval Agreement.

Members often acted in their own interests, for example, Britain did not close the Suez Canal to Italy during the Abyssinian crisis as they feared an attack on their navy.

Big bullies (powerful members) like Japan, Italy and Germany became more aggressive in the 30s as they began rearmament and invading weaker countries.