title: Airflow ExternalTaskSensor gets stuck
body: <p>I'm trying to use ExternalTaskSensor and it gets stuck at poking another DAG's task, which has already been successfully completed.</p>
  
  <p>Here, a first DAG "a" completes its task and after that a second DAG "b" through ExternalTaskSensor is supposed to be triggered. Instead it gets stuck at poking for a.first_task.         </p>
  
  <p>First DAG:</p>
  
  <pre><code>import datetime
  from airflow import DAG
  from airflow.operators.python_operator import PythonOperator
  
  dag = DAG(
      dag_id='a',
      default_args={'owner': 'airflow', 'start_date': datetime.datetime.now()},
      schedule_interval=None
  )
  
  def do_first_task():
      print('First task is done')
  
  PythonOperator(
      task_id='first_task',
      python_callable=do_first_task,
      dag=dag)
  </code></pre>
  
  <p>Second DAG:</p>
  
  <pre><code>import datetime
  from airflow import DAG
  from airflow.operators.python_operator import PythonOperator
  from airflow.operators.sensors import ExternalTaskSensor
  
  dag = DAG(
      dag_id='b',
      default_args={'owner': 'airflow', 'start_date': datetime.datetime.now()},
      schedule_interval=None
  )
  
  def do_second_task():
      print('Second task is done')
  
  ExternalTaskSensor(
      task_id='wait_for_the_first_task_to_be_completed',
      external_dag_id='a',
      external_task_id='first_task',
      dag=dag) &gt;&gt; \
  PythonOperator(
      task_id='second_task',
      python_callable=do_second_task,
      dag=dag)
  </code></pre>
  
  <p>What am I missing here? </p>