Most CMS packages allow users to control the <meta> tags, and it sounds like the software is overriding the server settings you're trying to use. So the solution is to find out how the CMS operates. Start with the documentation. If its well-supported package, the documentation should be available online. If it's not online, try contacting the vendor. If there's no documentation, then you'll have to try to use the package's Admin or Control Panel. If that fails, then you'll have to examine the source code for the package and figure out how it sets these tags.
Search engines are your best friend in these situations. Even if there's no official support, unless its custom software, you'll probably be able to find some references in online forums.