title: Airflow 1.9.0 is queuing but not launching tasks
body: <p>Airflow is randomly not running queued tasks some tasks dont even get queued status. I keep seeing below in the scheduler logs</p>
  
  <pre><code> [2018-02-28 02:24:58,780] {jobs.py:1077} INFO - No tasks to consider for execution.
  </code></pre>
  
  <p>I do see tasks in database that either have no status or queued status but they never get started.</p>
  
  <p>The airflow setup is running <a href="https://github.com/puckel/docker-airflow" rel="noreferrer">https://github.com/puckel/docker-airflow</a> on ECS with Redis. There are 4 scheduler threads and 4 Celery worker tasks. For the tasks that are not running are showing in queued state (grey icon) when hovering over the task icon operator is null and task details says: </p>
  
  <pre><code>    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
  </code></pre>
  
  <p>Metrics on scheduler do not show heavy load. The dag is very simple with 2 independent tasks only dependent on last run. There are also tasks in the same dag that are stuck with no status (white icon). </p>
  
  <p><em>Interesting thing to notice is when I restart the scheduler tasks change to running state.</em></p>