Airflow sensor doesn’t send email after missing sla

I have the below parameters for a sensor. My expectation is, as the schedule interval for the DAG is every hour at minutes 0, 20, 40, and the sla is 5 minutes, then if the sensor does not finish successfully, an email will be sent out at for example 5.05 pm , 5.25pm and 5.45pm. However currently the sensor is up for schedule and no emails were sent. Can someone please help to point out what I did wrong? Thank you.

dag= DAG(dag_id = "mydag",
         dagrun_timeout = timedelta(hours= 12),
         schedule_interval = "0,20,40 * * * *",
         start_date = datetime(2019, 10, 13, 15, 50),
         catchup= False,
         email= "myemail")

In sensor sla = timedelta (minutes = 5)

Source: Python Questions