Factors that contribute to the League of Nations' weakness
(Point) Limited membership
(Eg/Evidence) USA did not want to join (most Americans were isolationist), Russians refused to join (they were Communists and hated Britain and France) and Germany was not allowed to join until 1925
(Elaborate) USA was the strongest and most powerful country in the world and commanded great respect and political clout. Its inclusive in the League of Nations would have enhanced the league’s credibility, reputation and enforcement power. Its absence was a crucial missing link in the cornerstone of establishing peace in the 1920s. This was not a problem in the 1920s, when the League dealt mainly with small countries like Sweden and Finland (Aaland Islands, 1921) and Bulgaria and Greece (1925). But, without the three world powers, the League was too weak to make a big country do as it wished for instance, in the case of Lithuanian & Polish dispute in Vilna, LON was unable to force withdrawal of Polish troops & eventually Polish annexed Vilna. Also, Italy invaded Greece because of Corfu incident. The LON had to accede to Italy's request for reparations & an official apology from Greece before Italy agreed to pulling troops out of Greece.
(Link) This showed that the League was powerless against big nations as they could not command sufficient authority without the support of a big nation's military backing. Additionally, limited membership resulted in the League being weak because it did not represent the interest of the whole world, and hence it could not enforce actions if it had to on the world community.
(Point) Structural weakness of the LON
(Eg/Evidence) The parts of the organization did not contribute towards congruent functioning of the League.
(Elaborate) The Assembly could only make a decision by a unanimous vote and as because the unanimity impeded efficiency, the Assembly thus hardly never made any decision. The Council was the executive body