‘That will be because you have two processors’, was my first response.
Your application is stuck on one processor.  Either by a single thread looping, or Processor Affinity has been set.

Knowledge is knowing just a little more than the next guy/girl.

Which is why I read “The Old New Thing”.  I don’t understand it all, but I store the information away in case someone asks

Why is my application running at 50%?