C# Parallel.ForEach does not use the available CPU power

Question

I am running an application on a four core machine and it maxes out at around 14% CPU usage and stays there rather constantly.

Parallel.ForEach(pages,
    new ParallelOptions { },
      page =>
      {
         // do some CPU intensive stuff
      });

Only if I start several instances of the application do I get over 14%. By doing that I can get close to 100% in total.

Why doesn't Parallel.ForEach do the task parallel enough to use all of the CPU?

I am running C# .NET 4.5.1.