Auto-debits must be set up by the 18th of the month in order for them to run on the 20th.
If they're set up after the 18th they will only run the following month.
Where can I find the date?
You can confirm when your auto-debit will first run by checking the date on the auto debit card on your loan screen. You'll also see the date on the confirmation screen when you first set up your auto-debit.