Automation test wouldn't exit after tests finished

I created COMPLEX automation tests, and run it by command line.
The test runs good, and I can get the test result.
But, after all tests finished, the automation tool wouldn’t exit.
it always wait 30s and shows:
Client exited and has been quiet for 30 seconds…exiting

In the Log.txt it shows the log content:
…Automation Test Queue Empty %d tests performed.

But it not shown in Client.log
So I think somewhere the file stream or output was blocked/canceled.

Anyone can give me a help?