Besides having the correct permission set, the ability to resend notifications/webhooks requires the user to be in the account the envelope was sent from.
Below is an example:
The logged in user is currently accessing the top-level account (Training-Kelsey Solis). A search is performed within reporting which includes subaccounts.
The envelope requiring a webhook to be resent is highlighted below:
Please note the account which it was sent from – “Macaron Mischief”
Looking at the webhook notification, there is a lock next to it:
To resolve this, navigate to the subaccount that the envelope was originally sent from.
As seen below, there is now a resend option next to the webhook.