Some of the search options have additional limitations that you would not encounter when searching an EndNote library. For example, some databases prohibit searching for a year alone. The year can be specified only to limit a search (these are called "limit fields"). In such a case, a search for 1997 as the year would be denied, but searching for Smith as an author AND 1997 as the year (thereby limiting the results of the author search), would be allowed.
If you have set up a search that is not accepted by the online database, an error message will describe the source of the problem as specifically as possible. (See Troubleshooting Connections.) Keep in mind that these restrictions are set by the information provider, not EndNote.
Documentation from the information provider may help to clarify what search options are available.