Google Docs integration uses a different mechanism for storing citations than is used in the Word and LibreOffice plugins, and it can be easier for citations to accidentally become unlinked. The most common reason for 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. (The Zotero app itself is only required for inserting and editing citations.) Citations can also become unlinked if you drag them with the mouse within the Google Doc rather than cutting and pasting them.
To restore unlinked citations, you have two options:
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. Again, be sure to press Refresh after restoring to confirm that the citations aren't already unlinked in that version.
For information regarding other word processor plugins, see Existing Citations Not Detected.