https://www.vive.com/sea/support/ultimate-tracker/category_howto/cant-see-tracker-when-attached-to-lower-body.html
Why can't I see the tracker in VR when it's attached to my lower body?
When VIVE Ultimate Tracker is attached to a lower body part such as your ankle and you can't see it in VR, try the following:
see the trackerattached tolower bodyvr