Zotero can't automatically capture citation info from some web pages, but you can still add them to your Zotero library.
Right-click in your browser and choose "Save to Zotero" from the menu. This will save a new "web page" item to your library. You can add information about the author, etc., if you wish.
This will also attach a snapshot of the page to the citation. Taking a snapshot saves a copy of the page to your computer. It includes the page's text and images, so if the page is removed later, or if you're offline, you'll still be able to view your copy.