A knowledge base article about DUO 2-step push notification trouble provided by the UC Berkeley IT Service Hub - Knowledge Portal
You may have trouble receiving push requests if there are network issues between your phone and our service. Many phones have trouble determining whether to use the WiFi or cellular data channel when checking for push requests, and simply turning the phone to airplane mode and back to normal operating mode again often resolves these sort of issues, if there is a reliable internet connection available. Similarly, the issue may be resolved by turning off the WiFi connection on your device and using the cellular data connection.
Check the time and date on your phone and make sure they are correct. If the date and time on your phone are manually set, try changing your device's configuration to sync date and time automatically with the network.
Should neither of these actions help, see the Duo Knowledge Base for additional iOS and Android troubleshooting steps.
If you can't get Duo Push working on your own and your administrator has enabled Duo's device management portal, you can log in with a passcode generated by the Duo Mobile app and send a new activation link to your phone. See the My Settings & Devices guide for instructions.
If you've tried the suggestions here but can't get Duo Push working or reactivate your device yourself, please contact calnet-admin@berkeley.edu to request reactivation of Duo Mobile.