Alert rule is showing that the expression is satisfied. However Alert is not firing.

Here is the alert rule:

apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
  name: natgw-alert-rules
  namespace: {{ .Values.namespace }}
  labels:
    prometheus: k8s
    role: alert-rules
spec:
  groups:
    - name: natgw-alert-rules
      rules:
        - alert: NatGWReservedFIPFailures
          expr: |
            increase(
            nat_gw_errors_total{error_type="nat_reserved_fip_failed"}[5m]
            ) > 0
          #for: 1m
          labels:
            severity: medium
          annotations:
            summary: "NAT GW reserved FIP failure"
            description: "NAT GW reserved FIP failures are occurring in the last 5 minutes"