It's strange, that this method hangs or freeze for this method.
I used commons-net 2.2v, don't know if it already solved at 3v. But, i have found that my problem was:
Executing this method before closing the input stream.
Switching them, first closing the input stream and then running completePendingCommand() solved the issue.
There are some cases where this method isn't really necessary, beware.
Hoped it helped,