Webhook file upload notifications not received

We have created our webhook subscriptions according to the documentation provided in https://developers.sharefile.com/rest/webhooks/overview.aspx and confirmed the entities were correctly created in https://developers.sharefile.com/rest/docs/resource.aspx?name=WebhookSubscriptions.

We have subscribed to the following events:
"Events": [
        {
          "ResourceType": "File",
          "OperationName": "Upload",
          "odata.type": "ShareFile.Api.Models.SubscribedResourceEvent"
        },
        {
          "ResourceType": "File",
          "OperationName": "Update",
          "odata.type": "ShareFile.Api.Models.SubscribedResourceEvent"
        },
        {
          "ResourceType": "File",
          "OperationName": "Download",
          "odata.type": "ShareFile.Api.Models.SubscribedResourceEvent"
        },
        {
          "ResourceType": "File",
          "OperationName": "Delete",
          "odata.type": "ShareFile.Api.Models.SubscribedResourceEvent"
        }
      ],  

We have also confirmed our webhook url is publicly available to receive notifications using postman yet we are not receiving any notifications when we upload files to the specified folder.

Have we missed out any other configuration?