Unable to factory reset a TRV

After three of my Tado X TRVs became unreachable, and after some contact with Tado support which hasn't given me any help, I tried doing a factor reset. I was able to reset two of them, add them to the Tado app, and to Apple Home. But I can't reset the third one. I'm following the tap-and-hold instructions. Can anyone suggest what to do if this doesn't work?