What was one problem that undermined the league of nations’ effectiveness

Best answer to that question is likely this:  The League of Nations lacked strong leadership.

Explanation:

The United States never joined the League of Nations, in spite of the fact that an organization such as the League of Nations was the signature idea of US President Woodrow Wilson.  He had laid out 14 Points for establishing and maintaining world peace following the Great War (World War I).  Point #14 was the establishment of an international peacekeeping association. The Treaty of Versailles adopted that idea, but back home in the United States, there was not support for involving America in any association that could diminish US sovereignty over its own affairs or involve the US again in wars beyond those pertinent to the United States' own national security.   The lack of involvement by the world's fastest-growing superpower, the United States, hampered its effectiveness.

The League of Nations had set out clear goals for what it intended to do.  The main aims of the League were disarmament across nations, preventing war through collective security of the international community, settling disputes between countries through negotiation, and improving welfare of people around the globe.  But it proved unable to meet those goals.  The United Nations today has similar goals, and has been more effective in its efforts -- though there are still plenty of people who criticize the UN's effectiveness.