Sending Prometheus alerts to Slack

Hi everyone, I am studying alerting and monitoring with prometheus, and I setup alerts to slack. I get alerts when my ec2 instances are down, but I get none when the are back up even though I set send_resolved: true . Please can someone point me to what I might be missing?

This is the rule:

- name: Down
  - alert: InstanceDown
    expr: up == 0
    for: 1m
      severity: 'critical'
      title: 'Instance {{ $labels.instance }} down'
      summary: '{{ $labels.instance }} of job {{ $labels.job }} has been down for more than 1 minute.'
      severity: 'critical'```
Sorry I couldn't post text, slack won't let me.