You can return to the failed delivery later to re-attempt delivering it.
To do so, select a failed delivery from your current tour that you are going to re-attempt, press and hold on it. You will see the message asking if you would like to reopen it. Tap OK, and the delivery will become active again.
Once you reopen a delivery, you can try to deliver it again and mark it as Complete if it succeeds or Unable if it fails again.