Events+ Google Login Issues

I have a new installation of Events+ and I have just discovered that users cannot login via google to RSVP to events. A pop-up window appears showing a Google account page (if logged into their account), then the page disappears, but no RSVP is logged.

In events + settings there are API settings for Facebook & Twitter login for RSVPs, but nothing for a Google login...

Any ideas where to start looking to troubleshoot and fix this?