What should I do if any of the controllers is not tracked?
Try the following methods to troubleshoot the issue:
- Make sure that the controller is placed inside the play area.
- Turn the controller off and on again to reactivate the tracking.
- Restart the SteamVR app. If you still get an error, reboot your computer and reopen the SteamVR app.
For more information, you may refer to the 
SteamVR Troubleshooting page.