For one reason or another, the app you are trying to connect to will be down. When this happens, we will try to send data but it cannot be received on the other end. You can do a few things to see if this is the case:
Attempt the load the Webhook URL into your internet browser. If it has a "not found" error, then that endpoint is down.
Check your URL at https://dnschecker.org/. If your endpoint is down, you will see it as "Not Resolved" on the world map.
You can also confirm your endpoint is down by looking at the "Failed Relays Report" guide.