There are too many versions of the lyrics! But the basic premise is always the same. Long ago, young girls picked all the flowers. These girls have since grown up and married. Their husbands have fought in the war, died and been buried. But now we can't find their graves, because flowers have grown over them. But this just brings us back to the question of where said flowers are.
You could look at it another way, in which the final verse provides an answer to the original "where have all the flowers gone?" question: they're now over the graves of deceased soldiers.
There are too many versions of the lyrics! But the basic premise is always the same. Long ago, young girls picked all the flowers. These girls have since grown up and married. Their husbands have fought in the war, died and been buried. But now we can't find their graves, because flowers have grown over them. But this just brings us back to the question of where said flowers are.
You could look at it another way, in which the final verse provides an answer to the original "where have all the flowers gone?" question: they're now over the graves of deceased soldiers.