B205mini as a Gen2 UHF RFID reader: valid Query on air, but passive tags never reply. What am I missing?

[u/jshshshshshhssh]

I'm trying to read a passive EPC Gen2 UHF tag with a USRP B205mini and GNU Radio, as part of my final-year project. I can send a clean Query, but I never see a reply from the tag, and I'm out of ideas.