crawled data can not be queried in athena

1. Use glue to crawl data from s3 file which contains following json

{"a":"b", "c":"d"}
{"a":"b1", "c":"d2"}

2. The crawler successfully create a table in a database with all the columns recognized
3. Go to athena to query this table, query succeeded, but returns a 0 results.

Why does it return 0 results while it should have 2? At the same time there is no error at any step, and the schema is correctly analyzed.