Okay, the "proper" way to do it is with liver of sulpher. But most people don't have access to it. Vinegar should tarnish it just fine, but I've never used it on brass so I'm not sure what color it will turn. Salt and vinegar really does the trick, especially if you heat it with your torch as you go. Also, if you want a really good patina you should make sure your metal is really clean. You may be having a problem because often times brass parts are packaged with a protective coating that prevents tarnishing. Hope this helps!