The Duo iOS application was updated in August 2015 to be made reliable with recent iOS changes to its cryptography libraries. Therefore, we recommend using the App Store to update the Duo app if you see intermittent "Registration Required" when using Duo Push.
Please note that you might also see "Registration Required" messages if your phone was not completely set up with Duo, if you started and did not complete a reregistration, or if your phone otherwise lost synchrony with Duo's servers. If any of these are the case, then you can use Passport's (https://passport.gatech.edu) Two-Factor tab to "Setup Duo app on this phone" and reestablish synchrony. Note that since your App is out of sync, you might have to use a call or text message to access Passport.