Can't create custom queries and resolvers in nested fields on GraphQL/AppSync Amplify

Hello! I am trying to custom a query with graphQL amplify without success. I am able to use the default queries and mutations but I am not being able to create a query type Query to filter nested fields. For example, I want to get the Books that have difficulty = Beginner.

My schema in schema.graphql:

type Books @model {
    bookName: String! @primaryKey
    EN: BookDetails
    ES: BookDetails
}

type BookDetails {
    bookName: String
    bookDescription: String
    difficulty: String
}

My query in schema.graphql:

type Query {
  getBookByDifficulty(difficulty: String!): [Book]
}

I am unable to put this query to work with custom resolvers. I wrote req.vtl and res.vtl files, put them on the resolvers folder in the amplify backend folder but it doesn't work. I can see the query on the console, but it always returns null.

Query.getCocktailByDifficulty.req.vtl in amplify/backend/api/{API_NAME}/resolvers/:

#set($tableName = "Cocktail-4oiehkpc4bhpbiqxd6oj63bxqe-dev")

#set($queryExpression = {
  "expression": "EN.difficulty = :difficulty",
  "expressionValues": {
    ":difficulty": $util.dynamodb.toDynamoDBJson($ctx.args.difficulty)
  }
})

{
  "version": "2018-05-29",
  "operation": "Query",
  "index": "yourGSIName",  
  "query": $queryExpression
}

Query.getCocktailByDifficulty.res.vtl amplify/backend/api/{API_NAME}/resolvers/:

#if($ctx.result.items)
  #set($transformedItems = [])
  #foreach($item in $ctx.result.items)
    #set($transformedItem = {
      "id": $item.id,
      "name": $item.name,
      "EN": {
        "difficulty": $item.EN.difficulty
      }
    })
    $util.qr($transformedItems.add($transformedItem))
  #end
  $util.toJson($transformedItems)
#else
  $util.error("No items found", "ItemNotFoundError")
#end

Please please heeeelp! I don't find any useful documentation on the internet.

Thank you