You should not be using query_posts at all.
It should be noted that using this to replace the main query on a page
can increase page loading times, in worst case scenarios more than
doubling the amount of work needed or more. While easy to use, the
function is also prone to confusion and problems later on. See the
note further below on caveats for details.http://codex.wordpress.org/Function_Reference/query_posts (emphasis mine)
Second, the showposts argument has long been deprecated.
Third, the tag argument is not listed as deprecated, the source appears to still respect the argument, and it works when I try it. Make sure that you are using the tag slug and the name. The two may not be the same.