title: Airflow scheduler not pick up tasks, task waiting forever
body: <p>I met an issue that my task in a tag never got pick up by workers for some reason.
  When I look at the task details:</p>
  
  <blockquote>
    <p>All dependencies are met but the task instance is not running. In most
    cases this just means that the task will probably be scheduled soon
    unless:
    - The scheduler is down or under heavy load</p>
    
    <p>If this task instance does not start soon please contact your Airflow
    administrator for assistance.</p>
  </blockquote>
  
  <p>I checked the  scheduler, no errors in the log, also restarted it a few times.</p>
  
  <p>I also checked the airflow websever log, only notice this:</p>
  
  <blockquote>
    <p>22/11/2018 12:10:39[2018-11-22 01:10:39,747] {{cli.py:644}} DEBUG - [5
    / 5] killing 1 workers 22/11/2018 12:10:39[2018-11-22 01:10:39 +0000]
    [43] [INFO] Handling signal: ttou 22/11/2018 12:10:39[2018-11-22
    01:10:39 +0000] [348] [INFO] Worker exiting (pid: 348)</p>
  </blockquote>
  
  <p>Not sure what happens, it worked fine before.</p>
  
  <p>Airflow version 1.9.0, never change the version, only playing around some of the config: <code>min_file_process_interval</code> and <code>dag_dir_list_interval</code> (but I put it back to default when encounter this issue)</p>
  
  <p><a href="https://i.stack.imgur.com/UiHgm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/UiHgm.png" alt="enter image description here"></a></p>