It never fails to astound me: The amount of digital professionals who think they know more about a client's business and what's best for them, than they believe the client knows themselves.
It is a syndrome I have witnessed mostly afflicting digital startups.
Most of the time, what (good) digital agencies have to say is extremely valuable and will indeed offer excellent ROI. But sometimes, just sometimes. You need to accept that the client probably does know a little more about their sector and business then you do, after all, they must be reasonably successful to afford your rates right?
It is a difficult, but important skill: to distinguish between when you need to shout the client down and when you need to listen. But if you don't figure it out quickly, you won't last long.