title: Why an activity task is scheduled but not started?
body: <p>I have an activity task scheduled but never executed(started) by looking at the history, and then workflow got timeouted.
  I can confirm that cadence worker is running because other workflow works fine at the same time.</p>
  <p>Why the activity is not started/executed in history? How should I investigate issue like this?</p>
  <p>My activity timeout is same as workflow timeout.</p>
  <p>The question is from by this <a href="https://github.com/uber/cadence/issues/3802" rel="nofollow noreferrer">Github issue</a>.</p>