Failed to set Body Tracking fidelity to: High (IOBT)

Please help! I have read over this page many times, and done everything I can think of:

https://developer.oculus.com/documentation/unity/move-body-tracking

But when my app runs, adb always shows "iobt": false and I get IK body tracking instead. In my logcat I see:

Info XR_FB_body_tracking ------------ xrCreateBodyTrackerV2FB -----------
Error OVRPlugin [XRCMD][failure] [XR_ERROR_RUNTIME_FAILURE]: xrRequestBodyTrackingFidelityMETA(m_xrBodyTracker, fidelity), arvr/projects/integrations/OVRPlugin/Src/Util/CompositorOpenXR.cpp:8151 (arvr/projects/integrations/OVRPlugin/Src\Util/CompositorOpenXR.h:302)
Warn Unity [OVRBody] Failed to set Body Tracking fidelity to: High
Warn Unity OVRBody:StartBodyTracking()
Warn Unity OVRBody:OnEnable()

It appears BodyTracking is failing to initialise to 'high' fidelity (IOBT) but I don't know why? How can I debug this? Can I find out what CompositorOpenXR.cpp:8151 is doing?