Always confirm from the Duo Admin Panel Logs for the user
https://admin.duosecurity.com/login?next=%2F
If the location of the device is unknown in the entry that means it was sent but not received by the device for one of the following reasons below. If there is not an entry that means they were not utilizing the correct credentials or (least likely there is an issue with Duo Servers)
From the Duo log it appears as if your device is not receiving the push, not that it isn't being sent out. This can happen when;
The mobile device is using WiFi /Cellular and the local network is blocking this traffic or not available
Try switching from Wifi to cellular or vice versa and see if issue remains
The mobile device has power save features that prevent the Duo App from accessing WiFi or cellular network traffic
Verify the device does not have any power restrictions on the Duo Mobile app
The mobile device has security software or features that prevent the Duo App from accessing WiFi
verify any security apps that may be running in background
The devices current firmware is not up to date and creating a disconnect for the Duo App
Verify Device is running latest security updates/firmware and restart mobile device
