Thread:Project:Support desk/ subst:xxx not working

Hello, I searched and searched but found no answer. In my MediaWiki 1.16.0 the command is not working. If I use it, the command don't gets replaced by content of "xxx", on the site again "" is printed.

I only found this site for information to this command: http://meta.wikimedia.org/wiki/Substitution but there is nothing written that I have to do something that it works in my MediaWiki.

So my question: Is there something I must do that it works? If it should work by default: What can I do for debugging?

Thanks, Ticka