What is the reason for the React page not being re-rendered when setState is called?

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This can happen due to various reasons, some of the most common ones are:

1. The state value being set is the same as the existing one, so React skips rendering
2. The component where setState is called is not subscribed to that state value, so it doesn't re-render
3. There may be an error in the code that prevents the component from re-rendering properly
4. The setState may have been called outside of the React lifecycle events.

To identify the issue, developers can check the component's lifecycle methods such as shouldComponentUpdate, componentDidUpdate or componentWillUpdate, and verify how the setState function is being called. Additionally, they can consider using React Developer Tools to see the state and props of the component and their changes as they happen.