vox-tag-detector requires camera calibration, path to which which is stored here in the configuration file : /etc/modalai/voxl-tag-detector.conf . If you have not performed calibration of your tracking camera, please do that: https://docs.modalai.com/calibrate-cameras/
After that, make sure the voxl-tag-detector.conf is set up to listen to the correct camera (input_pipe should match the name of the camera that you are looking at through voxl-portal, typically tracking for the tracking caemra). Also make sure the detector is enabled "enable": true in voxl-tag-detector.conf .
Finally, as Tom suggested, if the tag detector encounters some error, it will display it in the log. You could just run the tag detector in the foreground by stopping the background service first and then running the detector in the terminal:
systemctl stop voxl-tag-detector voxl-tag-detector