How to debug when the search result doesn't return any highlight objects?

Elastic Stack Elasticsearch
1

I have the following search structure payload

GET es_index1,es_index2/_search?pretty
{
  "query": {
    "function_score": {
    }
  },
  "highlight": {
    "fields": {
      "summary.analyzed": {},
      "detail.analyzed": {}
    }
  },
  "timeout": "21s",
  "_source": false,
  "size": 8,
  "from": 0
}

For some query texts, the results don't return any highlight even though they contain the query text. If the highlight object I sent is correct, then how to debug this? On what cases the highlight may not exists in the result?

2

can you create a reproducible example with maybe a single document, where highlighting fails? plus index creation, plus mapping configuration?

That would be awesome!