Google Docs integration features a different mechanism of storing citations than Word or LibreOffice that makes the citations easier to accidentally unlink. The most common reason for citation unlinking is collaborators' editing the document without having the Zotero Connector installed. To ensure Zotero citations stay linked, everyone editing the Google Doc should install the Zotero Connector in their browser.

To restore unlinked Google Docs citations you may:

1. Use the Google Docs version history and restore to an earlier version of the document before the citation was unlinked. The version history can be found under Google Docs → File → Version History → “See version history”.
2. Use the Zotero plugin to reinsert the missing unlinked citations.

If all your collaborators have the Zotero Connector installed but you still see citations becoming unlinked, try to identify the actions that lead to the links breaking via the Google Docs version history and post details to the Zotero Forums.

For information regarding other word processor plugins, see Existing Citations Not Detected.