Adding up the last two years of Forrester research into the product management/product marketing role in the tech industry, it's easy to see why it can be hard to hire a good PM. Many of the skills that define successful PMs aren't easy to detect in a few hours of interviews. Since these traits generally don't reveal themselves until the PM is on the job, perhaps you should simulate the job in the interview.
While many technology vendors have a muddled idea of what PM success looks like, they usually have some notion of the outcomes they don't want to see. Can't build good working relationships with Development? Black mark. Can't say no to a sales enablement request? Black mark. Can't manage competing inbound and outbound priorities? Black mark. Can't give a detailed description of the specific roles to which we're marketing and selling? Black mark.
And the list goes on. The candidate sitting on the other side of the desk might have done an outstanding job, as it says on the resume, launching products at various companies. But what does "launching products" mean, exactly? Did the candidate keep the project on track, or was he just someone in the congratulatory photo op at the end? When you're interviewing a developer or QA person, you have a pretty good idea what their contribution was. The exact contribution of PM can be a bit more obscure, though certainly no less important.