Why does the instantiation of a prefab work well when the scene is first loaded, but then the cloned GameObjects are no longer displayed in the scene?

I made a GameObject, that dublicates the higher the Level is. The first time, it workes excelent, but when you lose the game and it is realoading the scene (with the level, you had bevore), the clones were still in the scene, but you can' t see them. Can you help me?