Tvheadend

Hi. Trying to get TVH (4.2.6-42 on Ubuntu 18.04) working.

I can't get the US ATSC muxes to pass the 'scan' stage. The tuner hardware is an HDHomeRun; the signal strength is grand (81 to 94%, depending on the station), but all the muxes perpetually FAIL the scanning step.

How do I troubleshoot this problem? Yes, I've tried restarting (everything..). Yes, the hardware is fine, yes the firewall on the Ubuntu box is receiving the traffic from the HDHR. The tuner streams play live like a dream in VLC.... Yes, I have tried every variation on the broadcast frequencies I can find (tangent: it's totally unclear what the most recent set of pre-defined frequencies for ATSC is and why the project would be shipping frequency sets it knows are bad is baffling), including looking them up on TV database sites to triple-check. Yes, the tuners are Enabled. It's literally just FAILing tests for reasons that it's not reporting.

All the logs ever show is
2018-11-13 12:25:39.338 mpegts: 506.31MHz in atsc-attic - scan no data, failed
2018-11-13 12:25:39.338 subscription: 7AF1: "scan" unsubscribing

...and so on. What else is there? If I could just manually tell TCH to use the mux, we'd be sitting pretty, but apparently it has to walk its way through this scan process even if I know what the muxes are, I guess? What can I do?