Why Kibana alerts are not consistent?

I have Kibana alert where I am NOT getting all alerts. This is inconsistent behavior where it works some times and sometimes not. I am not sure why.

Based on my condition I am checking certain status every minute (now - 1m). Kibana had 5 incidents that fits this criteria but only received for alerts.

FYI, this alerts are being sent to Slack channel. Below is the case where I have 5 events but only 4 alerts.

Hi,
Can you paste your watch here( obfuscate the sensitive information) . Also the Kibana logs would be helpful too and the version of the stack you are using.

Thanks
Rashmi

Kibana Version: 7.1.1 Management

Query:

  {
        "size": 1,
        "timeout": "30000ms",
        "query": {
            "bool": {
                "must": [
                    {
                        "match_phrase": {
                            "environment": {
                                "query": "prod",
                                "slop": 0,
                                "zero_terms_query": "NONE",
                                "boost": 1
                            }
                        }
                    },
                    {
                        "match_phrase": {
                            "service": {
                                "query": "tce-order",
                                "slop": 0,
                                "zero_terms_query": "NONE",
                                "boost": 1
                            }
                        }
                    }
                ],
                "filter": [
                    {
                        "range": {
                            "@timestamp": {
                                "from": "now-1m",
                                "to": "now",
                                "include_lower": true,
                                "include_upper": true,
                                "format": "epoch_millis",
                                "boost": 1
                            }
                        }
                    },
                    {
                        "match_phrase": {
                            "data.dimensions.status": {
                                "query": "FAILED",
                                "slop": 0,
                                "zero_terms_query": "NONE",
                                "boost": 1
                            }
                        }
                    }
                ],
                "adjust_pure_negative": true,
                "boost": 1
            }
        },
        "version": true,
        "_source": {
            "includes": [],
            "excludes": []
        },
        "stored_fields": "*",
        "sort": [
            {
                "timestamp": {
                    "order": "desc",
                    "unmapped_type": "boolean"
                }
            }
        ],
        "aggregations": {
            "2": {
                "date_histogram": {
                    "field": "data.timestamp",
                    "time_zone": "America/Los_Angeles",
                    "interval": "10m",
                    "offset": 0,
                    "order": {
                        "_key": "asc"
                    },
                    "keyed": false,
                    "min_doc_count": 1
                }
            }
        }
    }