With two factor authentication it is very important that the time on our server and your mobile device and the time within your Google Authenticator app is in sync, therefore please try to re-sync Google Authenticator by going to Settings -> Time correction for codes -> Sync now, or if you use iOS please set; Settings -> General -> Date & Time -> Set Automatically -> to On, and then make sure your mobile device displays the correct time and then try again.
Also, here's our two factor authentication guide for more info:
https://www.bitstamp.net/s/documents/bitstamp_2_factor_authentication_guide.pdfIf you are unable to resolve your issue please contact us at
support@bitstamp.net.