Hey folks - had the same problem and contacted Fi support. Here is the email they sent me to troubleshoot the fingerprint sensor. Hope it helps!
1. Remove all previously enrolled fingerprints:
Go to Settings > Security & location > Pixel Imprint and enter your PIN / passcode.
Next to all enrolled fingerprints, tap Delete .
2. Add a new fingerprint:
- Go to Settings > Security & location > Pixel Imprint and enter your PIN / passcode.
- Tap Add fingerprint, then follow the prompts to scan your fingerprint.
- Note: When you scan your fingerprint, hold your phone in the same way you would hold it when you're unlocking your phone. For example, hold your phone with the screen facing you.
- Repeat steps a-b to add more fingerprints.
3. Improve fingerprint recognition (this worked for me):
- Go to Settings > Security & location > Pixel Imprint and enter your PIN / passcode.
- For each enrolled fingerprint, touch the fingerprint sensor with that finger 20 times, shifting your finger slightly each time. This will improve your device's ability to recognize your fingerprints. (all you need to do is touch the sensor over and over again with the registered fingers on your device)
4. In the event the troubleshooting steps above don't resolve the issue, follow the steps within this article called Reset your Pixel or Nexus to factory settings.
- Repeat the previous steps to add a new fingerprint.
Just in case completing a factory data reset and repeating the steps above on the device doesn't resolve the trouble, our next step will be to move forward and replace the device.