Why aren't changes I make syncing between multiple computers and/or zotero.org?
Most likely, 1) you're looking in the wrong place, 2) the sync process hasn't completed, 3) you've received a sync error on one of your computers, or 4) you are using multiple Zotero accounts.
1) Looking in the wrong place
In Zotero libraries, all existing items are shown in either the library root or the trash. Make sure you're not looking in a collection that contains only some of your items. If you don't see the left pane in Zotero, click the bar at the left edge of the window to show it. If you don't see your collections listed, make sure the library is expanded by clicking the arrow or plus sign next to the library. If you don't see a group, check that it's set to sync with your client by clicking the “Choose Libraries…” button in the Sync pane of the Zotero preferences. If you don't see a group on the Zotero website, make sure you're looking in the Groups tab at the top of the page.
2) Sync process hasn't completed
If you're sure you're looking in the right place, check to see whether the data in question has synced to your online library on zotero.org. If you don't see the data in the online library, the problem is on the computer where the data originated. Check the green sync icon in the Zotero pane on the relevant computer. If it's still spinning, the sync process hasn't yet completed. Hover over the sync icon to see the current status.
3) You've received a sync error
If you see a red error icon to the left of the sync icon, an error has occurred during the sync. Click the icon to view the error. It may give you enough information to fix the problem yourself, or you can report it and post the Report ID to the Zotero forums for further assistance.