[F]rench and British self interest
They looked after their own interests rather than the League's.
1930s - Hoare-Laval Pact in Abyssinia (they would rather have given away another Country's land than lose Italy's support in the struggle against Hitler).
1930s - Lytton Report (Manchuria) brought them time instead of stopping Japan so that Britain and France would not have to defend China.
1920s - Vilna - France wouldn't support the League against her ally Poland ... Poland kept Vilna and France kept her ally against Germany.
Key countries (esp. USA) weren't members of the League of Nations.
1930s - In the Abyssinian crisis the USA carried on selling to Italy because she was not affected by the League's ruling (imposing sanctions). This meant the sanctions were even less effective.
1930s - After each crisis (Manchuria, Abyssinia and Disarmament) a major power left the League which weakened it; there were more powerful countries outside the League than in it.
1920s - Germany not being in the League made it somewhat ineffective - most of the major problems revolved around Germany.
Sanctions imposed by the League were either ineffective, easily broken or often no imposed at all.
1930s - In Manchuria sanctions were not imposed on Japan until AFTER she had taken the province and invaded Jehol (the next one).
1930s - In Abyssinia sanctions were on items that weren't vital to Italy's invasion (such as Tin). PLUS Britain and France didn't close the Suez Canal.
1920s - No sanctions in the 1920s - Not for Italy's invasion of Corfu or for Poland's invasion of Vilna.
[L]ack of armed forces
The League had no troops of its own.
1930s - In Manchuria the closest powers were Russia and America (not members of the League!). Britain had forces in Hong Kong but was not prepared to let the league use them (self interest).
1930s - In Abyssinia Britain had forces all around the colony but didn't use them!
The League's job was to enforce the treaties made in 1919/1920 despite the fact most people now though they were unfair.
1930s - German rearmament was allowed in 1932 which ...
* Began the dismantling of the Treaty of Versailles. It undermined the League's aims (DIES[E])! Britain did this because she believed the TofV was unfair.
* The Rhineland was remilitarised, France didn't react because Britain believed the TofV was unfair.
* Hitler's next step was to take Austria (the Rome-Berlin Axis) and the British didn't back up France because they believed the Austrians and Germans should be self-determinate.
1920s - Reparations were gradually (1924-29) reduced and German joins the League in 1926.
[R]eached decisions slowly
The League took a long time to act (either deliberately or by design)!
1930s - Manchuria - The Lytton report took a year; it was a way of stalling in the hope that the incident would be forgotten.
1930s - Abyssinia - The investigation of the original incident at Wal Wal didn't stop Mussolini shipping his army to East Africa.
1920s - The Disarmament Conference was constantly put off.
1930s - Both Manchuria and Abyssinia were invaded to get resources (the lack of which was due to the economic crash).
*Sanctions were often imposed - Britain kept selling Italy coal to save coal miners jobs in Britain. Or they were easily broken because, if one country didn't sell goods, another was keen to! (e.g. USA to Italy during Abyssinia).
1920s - No sanctions were imposed on Italy over Corfu! Boom time - trade was okay and there were lots of people to trade with.