A couple of us had a similar experience and it was due to entering incorrect information into the "Device Name" text field. We interpreted this as naming the headsets we were enrolling, but when trying to enroll the headsets the MDM would not scan the QR code or give any type of notice that the field was incorrect. Any other field will prompt you to correct your inputs
Make sure to enter your device type into "Device Name" (VIVE Focus 3 for example)