Off the top of my head, the only thing I can think of is are you using any sort of proxy, such as NGINX, or firewall that may be blocking the incoming POST traffic?
And just to check are you also sure you’re doing something to trigger notifications to be sent in the first place? For example turning the stream off and on again wont do anything if you do it too quickly as there is a delay of several minutes before notifications are sent and if it’s just a small outage then it may not trigger any notification to be sent. Changing game or title should work to but again will be delayed by several minutes.