Why aren't changes I make syncing between multiple computers and/or zotero.org?
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, or go to View → Layout and make sure Collections Pane is checked. If you don't see your collections listed in the left-hand pane, make sure the library is expanded by clicking the arrow or plus sign next to the library.
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, and any other computer is irrelevant. Check the green sync icon in Zotero 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 post to the Zotero Forums for further assistance. Be sure to include the message you're receiving and a Report ID in your forum post.
4) The library isn't set to sync
In the Sync pane of the Zotero preferences, click “Choose Libraries…” and make sure that the library you're trying to sync has a checkmark next to it. If not, click in the Sync column to enable syncing for that library. All libraries are set to sync by default.
5) Check your Zotero account details
Check the Sync pane of Zotero preferences to make sure you're using the same account you're using to log in to zotero.org and on each of your computers. If the problem is with a group, make sure the account is a member of that group on zotero.org.
6) Further troubleshooting
If you're still having trouble, post to the Zotero Forums with a Debug ID for the first sync after making a change that doesn't sync (e.g., making a change to the online library that doesn't appear in Zotero, or vice versa). Start debug output logging before making the change and keep it going until the sync has ended.