We haven't changed our Google authentication code recently, since it perfectly serves tens of thousands secure sign ins every month.
Google lists several common possibilities on their site so take a look. We've also pasted some of the useful excerpts here for you.
Some of your third-party cookies or data are blocked
When enabled, this privacy feature deactivates all cookies and storage within the iframe, which is required by Google to securely authenticate you. If you have disabled third-party data, please create an exception for https://accounts.google.com by adding
accounts.google.com to the allowed domains. In Chrome, this is done in chrome://settings/content/cookies.
You use Chrome on iOS in incognito mode
Google Sign-in is currently not supported in incognito mode on Chrome on iOS.
You use Safari with Intelligent Tracking Prevention (ITP)
This new feature of macOS High Sierra and iOS 11 deactivates third-party cookies every 24 hours, unless you interact with one of Shake pages. As the Google Sign-in library relies on cookies to securely authenticate you, it will potentially detect that you are logged out every 24 hours. You have to go through the sign in flow again to reactivate the cookies.