Sometimes the icon for a book/article/etc doesn’t appear in the address bar right away. What’s going on?
Zotero must wait for the entire page to load before it can check for an item on the page. Sometimes there are elements of the page (a large image, e.g.) that take time to load and thus delay Zotero’s activity.
If the page has finished loading, but you still don't see the proper item type icon, see here.