First, check that your device is still connected with the app. If the device is successfully paired and connected, you will see a connected symbol at the top-left of the device screen.
Log in to the app and visit the Profile screen to confirm the device is successfully connected (at the top of the page).
If these are both working correctly, follow the troubleshooting steps under My Delta will not sync.
If this still does not resolve the issue, try force-closing the app and logging out of the app and back in again.