I've set up repeating payments (CiviCRM 5.21 with Drupal 7.69) with Authorize.net for a client.
In Civi, the transaction hangs at "Pending".
At Auth.net, everything looks good, the initial payment has been processed with a date set for the next transaction.
Also note that one-off transactions from the site process perfectly with the transactions showing as "completed" in CiviCRM.
Not sure on how to troubleshoot this.
I thought that @Eileen's notification log extention might help. However, the system log table is empty.
I'm not sure what the next step is for troubleshooting.