Please check if your callback URL accepts POST requests. It should also return 200 OK HTTP response or else webhook will be considered as “failed” and after several attempts will be dropped.
If a particular e-commerce platform does not support native webhooks (e.g Magento, Opencart), webhooks functionality is emulated by querying store data periodically. Currently, this period is set to 300 seconds, so you can expect 5 minutes delay when receiving webhooks from some platforms.
There is a special API method account.failed_webhooks that allows you to list recently failed webhooks. Please note that information about failed webhooks is stored for 24 hours.