If a check has the status message "Warning: whole content filtered (please verify your filter settings)", then there is probably something wrong with your filter definitions and WebSite-Watcher isn't able to detect updates.
Usually WebSite-Watcher compares the filtered text content of the new version with the filtered text content of the old version of a page. If the whole content is filtered, then WebSite-Watcher has nothing to compare and is not able to detect updates. In such a case this error message is displayed.
Open the bookmark properties and verify/change your filter definitions to fix that behavior. With the feature "Test filter" you can see the content which is compared to detect updates.
See also related help topic at https://www.aignes.com/wswhelp/help_whattodo_wholecontentfiltered.htm
If you don't have filters defined, then the page might not have text content what can also cause that error message.
Two further things you can try:
1) Open the URL in your web browser and see if it redirects to another URL. If so, try if the new URL works in WebSite-Watcher.