splunk does not start indexing even after add data with associated index

Hi, I have created S3 data input, and set it to a pre-created index, I am expecting splunk will automatically and incrementally index data under the input directory. But sometimes, nothing happens and the index is of size 0. Did I miss something? How do I make sure splunk starts indexing always and incrementally index the data I just added?

Thanks.

Tian