These are some of the most common issues we've come across when working with API implementers.
I'm not receiving any callbacks to my webhook implementation
When using a wrapper to your webhook implementation you may find the wrapper has it's own requirements which prevent calls showing up in your logs. In one example, an implementer was unable to view incoming messages arrived at their Azure API Management wrapper. This was caused by the message hitting the Azure Service Bus and being rejected as it already contained our HMAC Authorization header but Azure Service Bus added an SAS token to the request causing multiple Authorisation headers to appear.
In this case, attempt we found the problem making calls with an HTTP Rest client (such as Postman) and adding a test Authorization header before viewing the response.