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.
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 a specific library isn't being synced, check that it's set to sync 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.
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.
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.
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.
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). It may be helpful to start debug output logging before making the change, since Zotero may sync automatically soon after.