Thanks for the suggestions @BulldogLowell
I am using a one-minute interval. Connection status is good and other publish and subscribe functions are working properly. (I have a heartbeat function that also publishes every minute and watches for a response and it’s working perfectly.)
curl to the service works instantly and reliably.
When I implemented my first webhook I made a mistake in the spark code that called the webhook many times super quickly while the data wasn’t set. I’ve fixed this now but wonder if that got me blacklisted. 
Incidentally I get a timeout error now when I try to create new webhooks.