If a sync fails for some reason, the sync button will display a small red or yellow badge. Clicking the badge will usually allow you to diagnose and resolve the problem quickly.

If taking the steps suggested by Zotero do not help, you can also solve many syncing problems using the Reset tab in the Sync pane of the Zotero preferences.

The reset mode to use depends on your situation:

If all of your data is on the Zotero server, use Restore from Server.

If all (or nearly all) of your data is on one computer, use Restore to/from Server.

If you've made significant changes on multiple computers between syncs, use Full Sync.

First, check your Zotero library on zotero.org to verify that it contains all of your data. To view your library, log in and then click the My Library tab.

Next, if you have Zotero's auto-sync enabled, open the Zotero preferences without opening the Zotero pane. Go to Firefox's Tools menu, select Add-ons, and select the Extensions tab. Select Zotero in the list that appears and click the Preferences button.

Once in the Zotero preferences, click the Sync pane and the Reset tab within that. Finally, click “Restore from Zotero Server”.

Start on the computer containing the Zotero data you want to keep, and make sure Firefox is closed (or auto-sync is disabled) on the other computers.

If you have auto-sync enabled, open the Zotero preferences without opening the Zotero pane. Go to Firefox's Tools menu, select Add-ons, and select the Extensions tab. Select Zotero in the list that appears and click the Preferences button.

Once in the Zotero preferences, click the Sync pane and the Reset tab within that. Click “Restore to Zotero Server”.

Next, on each other computer you want to sync, open Firefox and, using the same method as above, access the Zotero preferences without opening the Zotero pane.

In the Zotero preferences on each computer, click the Sync pane and the Reset tab within that, and then click “Restore from Zotero Server”.

Your data should now be fully in sync across all your computers.

If you have libraries that differ on multiple machines, you can try the “Full Sync” option in the Sync→Reset tab of the Zotero preferences. A full sync ignores all sync history, pulling all data from the server and comparing it to all local data. If any of the same items are different, you'll see a conflict resolution dialog allowing you to choose which copy to keep.

While a full sync will often work, if you have made complex organizational changes on multiple computers it may not complete successfully. In this case, you'll need to use the Restore to/from Server + Manual Merge option below.

If a full sync fails, but you have data on more than one computer that you wish to keep, you may wish to do a “Restore to Zotero server” on the computer with the most recent data and then manually save any differing data on other computers.

On the other computers, if you have auto-sync enabled, first access the Zotero sync preferences without opening the Zotero pane. Go to Firefox's Tools menu, select Add-ons, and select the Extensions tab. Select Zotero in the list that appears, click the Preferences button, select the Sync pane, and make sure “Sync automatically” is not checked.

Next open Zotero and sort My Library in the middle column by clicking on Date Added. (If you don't see Date Added, click the square button to the right of the column header row and select it from the list.) Select the recently added items and export them to Zotero RDF. If you've added or modified notes attached to older items, you'll want to export those as well.

After exporting, open Zotero's Sync preferences, click the Reset tab, and choose “Restore from Zotero Server”. After restoring, import the RDF file(s) you export back in and sync normally with the server by clicking the green sync icon.

You will need to relink reimported items in any documents created with the word processor plugins.