Common issues and how to resolve them.
Almost always kernel limits. Confirm vm.max_map_count=262144 and the nofile limits (Install hardening), then restart the ClickHouse service.
External checkers only run when the flow threshold or a Custom Rule needs their signal. Add the checker to a rule, or lower the threshold, to force it.
Callbacks aren't sent to private networks. Ensure the callback_url is public (HTTPS) and the merchant key matches, so the HMAC verifies.