I would verify that is valid JSON in that file. Also run Logstash in --debug mode to see if it give you any other hints.
Are the JSON records 1 per line or multiline? Can you give a sample of how it's structured without using the actual data?
It should look like this with how your input is setup.
{"message": "value", "another": "field"}
{"message": "value", "another": "field"}
{"message": "value", "another": "field"}
{"message": "value", "another": "field"}
{"message": "value", "another": "field"}