Logstash thinks it has already processed the files so it's just waiting for more data to be appended to hem. Setting the sincedb_path option like that resets the saved state about the current position in the file, so that combined with start_position => "beginning" will make Logstash read the files from the top. Alternatively you can delete the current sincedb files to reset the state.
See the file input's documentation and countless previous threads on this topic.