When testing the endpoint, if Magento 2 shows the message "You have [number] unprocessed notifications". There is a problem with the system's cron job. The Magento cron does not only accept the notifications, but also has to process the notifications to update the status in Magento.
In order to resolve this, make sure that your cron job is running properly. More information on configuring and running the cron for Magento can be found in Magento's documentation here: Configure and run cron.
If the cron job is running, then check the
adyen_notification table if there are unprocessed notifications which are more than 5 days old. Notifications older than 5 days are not processed by cron, and you need to go in the table
adyen_notification in your Magento 2 database and update their creation date to less than 5 days.