For those hosting in WPEngine, please remember to request that they enable the Alternate Cron and select WP Cron option in RBE instead of Webhook.
Mariah’s issue is unrelated, and running the webhook manually (pasting it in a browser window) works. This usually means that there’s nothing set up to call the webhook automatically. Switching to WP Cron will work, or setting up a real cron job to call the webhook.
Closing this as fixed.