Zotero 1.5 is currently in beta, and it is possible you'll run into a bug in the sync process that could prevent you from syncing your data. You can solve most syncing problems using the Reset tab in the Sync pane of the Zotero preferences. Selecting “Restore from server” and clicking the Reset button will erase your locally stored library and replace it with the data stored on the server. “Restore to server” will do the opposite, replacing the server data with your locally stored library.

If a sync fails for some reason, the sync button will display a small yellow badge. While we encourage you to submit bugs and help us diagnose problems, if you just want your data synced and don't want to worry about what's causing the bug, first decide which copy of your data you want, server or local. If you have auto-sync enabled, you can avoid the possibility of it causing problems by opening the preferences window without ever opening the Zotero pane. This is done by going to Firefox's Tools menu and selecting Add-ons. Select Zotero in the window that appears and click the Preferences button.

Now that you have the preferences window open, go to the Sync tab and then the Reset tab within that. If the data you want to keep is on the server, select “Restore from server” and click the Reset button. Contrariwise, if the data you want is on your computer, select “Restore to server” and click Reset.

If you wish to confirm the contents of the server, log into your Zotero account on this site and click My Library in the top-right corner. This will display the library items that exist on the server. It may be worth doing this before and/or after trying to restore your data to make sure you know what data is going where.