Thanks for your suggestions, they put me on the right path.
I tried it in a different computer and confirmed that it worked in that one. I eventually realised that I had something installed that might be interfering with it - Interception https://github.com/oblitum/Interception
I'd been looking at adding chords to my (nontwiddler) keyboard, and installed interception and autohotkey interception libraries. I'm fairly sure that this issue: https://github.com/oblitum/Interception/issues/25 explains why it wasn't working - interception has a limited number of input devices and subsequent ones don't work.
After uninstalling interception and reconnecting, the twiddler works again.