If you or someone in a group you're a member of have used a newer version of Zotero on another computer, you may receive this warning when syncing Zotero:

“Some data in Zotero could not be downloaded. It may have been saved with a newer version of Zotero.”

When new features are added to Zotero, older versions of Zotero may not know how to handle the data required to support those features. To avoid problems, Zotero will refuse to download data it doesn't understand, while continuing to sync other data in your libraries.

You can click “Check for Updates” in the warning dialog to see if there's a newer version of Zotero available. If no newer version is available, the data may have been created in the Zotero Beta or the Zotero for iOS beta, in which case you will need to install the Zotero Beta to sync all data or ignore the warning until a newer version of Zotero is available on the main release channel.