When you test, are you staying online for some time, or just going online then immediately offline? Webhooks have several minutes delay to give time for all the API servers time to update so that any webhook notification will be consistent with the API. This also means that a stream going online then offline shortly after, or a stream that is already online having a brief outage, wont send notifications.
Without seeing your code, another point that sometimes causes people trouble is not realising the notification is a POST request, so make sure your handler is capable of dealing with that type of request rather than just a GET.