First make sure a dialog isn't just appearing behind Zotero or your browser window. That's the most likely issue here.

If you're sure it's not that, restart your browser and Zotero and try again, and also try disabling all other third-party browser extensions and reloading Google Docs.

If that still doesn't fix it, restart your browser and Zotero and provide Debug IDs from both the Zotero Connector and Zotero.