You've probably processed the file before and then Logstash will remember what you've read and only tail the end of the file. If you want the file processed from the beginning you have to
- delete its sincedb file (or set sincedb_path to null for the file input), and
- set start_position to beginning.
You can confirm that this is the problem by appending a line to the log (echo foo >> /home/user/data/trace_collector.log).