无法使用Amazon S3 PHP SDK删除对象。 可能的原因是什么？

I am using official PHP SDK of Amazon S3 with official service provider for laravel. I am using version 3. The following is my code for deleting my target object:

$client = AWS::createClient('s3');

$result = $client->deleteObject(array(
    'Bucket' => self::$bucketName,
    'Key'    => $dest
));

return $result;

And here is the return message:

Model Data
----------
Data can be retrieved from the model object using the get() method of the
model (e.g., `$result->get($key)`) or "accessing the result like an
associative array (e.g. `$result['key']`). You can also execute JMESPath
expressions on the result data using the search() method.

{
    "DeleteMarker": false,
    "VersionId": "",
    "RequestCharged": "",
    "@metadata": {
        "statusCode": 204,
        "effectiveUri": "https:\/\/s3-ap-southeast-1.amazonaws.com\/my-bucket-sandbox\/testing\/NjY0MA%253D%253D1481788705.jpg",
        "headers": {
            "x-amz-id-2": "UXlVnYeU0ID07N58C20G7yh5aKIdu7jj2QP8Ai8MZc+ay5KjAen4NHPA2oRONIAGyJXuE\/kw+Rs=",
            "x-amz-request-id": "EBC374A27EE974FF",
            "date": "Thu, 15 Dec 2016 08:05:47 GMT",
            "server": "AmazonS3"
        },
        "transferStats": {
            "http": [
                []
            ]
        }
    }
}

From the DeleteMarker I know the delete action has failed, but the reason is not provided in the return message. Here are some other clues:

1. As I previously successfully uploaded the image, I guess it is not because of permission problem.
2. And I guess I should have created my client with correct connection, else the upload should have failed.
3. It returns 404 when the key (or path?) of the image is wrong. As I receive 204 this time, I guess it is not because the key is wrong.

Any suggestion on what are the possible reasons?