It should work with parentheses. You should check the HTML output and/or your vortex.log file to see if there were any errors reported evaluating the expression. Also which version of Texis are you using?
We are using Commercial Webinator 3.01. The parentheses did work fine on our old server, but we are currently migrating everything over to an iPlanet Web Server running on Windows 2000. Thats when I noticed the problem.
I would check the vortex.log for error messages. Vortex does need to be able to open the current database to evaluate the more complicated expressions, and maybe that is causing the issue.