During non-peak times, funds are usually withdrawn on the payment date you specify, assuming your e-filed return has already been accepted (received) by the IRS/State
Due to high e-filing volume it can take a little longer to process the payment and it can take your bank another few days to post the debit to your account.
No worries; as long as you selected April 18, 2017 (or earlier) as your payment date, you won't get penalized for a late payment.
If it has been more than 10 days since your tax return has been accepted call IRS e-file Payment Services at 1-888-353-4537 or contact your state tax agency, to confirm the scheduled debit dates.