As your client I would ask you for evidence that the traffic you’re sending is in fact high quality.
And I would say as your client that if that evidence is provided to me áfter I’ve asked why the results are low, I think you are too late. At that point I already have doubts about your capabilities and question everything you say.
So as a media buyer proactively bring up your concerns with the client and provide data to back up your concerns. Provide examples of what competitors do differently and how you believe those differences will have an impact on performance.
I personally often talk about expectations set in the ads not being met on the website. I usually start that conversation by asking if the client has concerns about the ads. They know the market better than we do, do they believe the ads are setting the wrong expectations, are the ads showing the wrong things? It shows being open to criticism and willingness to make changes.
But it’s also a trick question, because clients always approve ads before they go live, so they have to either sort of admit they gave us wrong input or they have to be open to the idea that maybe there is indeed something that can be improved on the website.
Not that I want to screw or trick clients, but I go into a conversation with an outcome in mind and this helps the client come to the same conclusion I have already come to more easily. Your client wants results, but the next best thing is feeling informed and in control.