Spring Data results Empty Page

I am using Spring data Elastic Search for tags based image search. (Image is parent which has list of tags)

Parent Object

public class Image {

@Id
private String id;

@Field(type = String, index = not_analyzed)
private String url;

@MultiField(mainField = @Field(type = String), otherFields = { @InnerField(index = not_analyzed, suffix = "verbatim", type = String) })
private String title;

@Field(type = Nested,index = not_analyzed)
private List<ImageTag> tags;
<--- getters and setters -->
}

Nested childObject

public class ImageTag {

private String tag;

private Float score;
 <-- getters and setts
 }

Repository Class

public interface ImageRepository extends 
ElasticsearchCrudRepository<Image,String> {

Page<Image> findByTagsTag(String tag, Pageable pageable);

Repository always Return Empty page content

I am referring this GitHub link for my test

https://github.com/eugenp/tutorials/blob/master/spring-data-elasticsearch/src/test/java/com/baeldung/spring/data/es/ElasticSearchIntegrationTest.java

which works perfectly for the search of books by author names

Why in above code I am getting empty page content.