AlertManager isn't sending alerts to webhook

[Marc Boorshtein]

I have the following config for alertmanger 0.26.0:

global:
  resolve_timeout: 5m
route:
  receiver: "null"
  group_by:
  - namespace
  routes:
  - receiver: monitoring/critical-alerts/nginx-webhook
    group_by:
    - namespace
    matchers:
    - severity="critical"
    - namespace="monitoring"
    continue: true
    group_wait: 30s
    group_interval: 5m
    repeat_interval: 30s
  - receiver: "null"
    matchers:
    - alertname =~ "InfoInhibitor|Watchdog"
  group_wait: 30s
  group_interval: 5m
  repeat_interval: 12h
inhibit_rules:
- target_matchers:
  - severity =~ warning|info
  source_matchers:
  - severity = critical
  equal:
  - namespace
  - alertname
- target_matchers:
  - severity = info
  source_matchers:
  - severity = warning
  equal:
  - namespace
  - alertname
- target_matchers:
  - severity = info
  source_matchers:
  - alertname = InfoInhibitor
  equal:
  - namespace
receivers:
- name: "null"
- name: monitoring/critical-alerts/nginx-webhook
  webhook_configs:
  - send_resolved: true
    url: http://nginx-alerts.alert-manager-webhook.svc/webhook
templates:
- /etc/alertmanager/config/*.tmpl

I'm not receiving anything to my webhook.  I increased verbosity to debug but I don't see anything related to my webhook.  There are plenty of alerts with severity=critical.  What am I missing?

Thanks

Marc