In the example above, this is what is stored in the system configuration file. If you are using the GUI, you will need to create a Data Group List of name "nodes" and then only include the when part of the rule.
If you can explain further what you mean by "fails" we might be able to help further. A good debugging trick is to make use of the log statement to output values you are expecting to search on. In the case of this example logging the [HTTP::cookie "server"] value might help if you aren't finding an exact match.
-Joe