UI Component Not Rendering After State Update

1. State mutation instead of immutable state update (mutating object directly)
2. Using direct array operations (push, splice) instead of immutable alternatives (concat, slice)
3. Key prop missing or incorrect in list rendering causing elements to be reused
4. Setter function called but returns same reference due to incorrect comparison
5. Parent component not re-rendering when child receives new props

Never mutate state directly. Use spread operator or Object.assign() for objects, array methods like concat/slice for arrays. Add explicit keys to list items using unique identifiers, never array index. Use immutable patterns: setState(prev => ({...prev, field: value})). Verify dev tools show state changing. Add console.log in render to confirm render called.