Have you noticed that people can make sense of what does not make sense when they do not understand it? That is the "soft" and "analogic" property of our brain: we can make sense out of nonsense, and this is ok, when we do not know it is nonsense!
A waterfall process will take nonsense, and then often spend the money while delivering nonsense.
Agile process will take nonsense, and fail iterations, and... just go on, spend your money, and delivery nonsense, or..., recognise the stalling as a failure to deliver the requirements (e.g. sprint stories), and then force the requirements to be rewritten for the next development iteration. The next iteration may fail, but again the requirements will need to be rewritten. Yet if you are truthfully applying an agile process, each failed delivery will again attempt to reformulate the requirements to be achievable and in the end what was not understood and is blocking you becomes understood and you either stop the project, or you understand how remove the blocker.