Solved
Failed to convert recording, because another recording was still converting
When I finished my meeting I got an error message to say the recording failed to convert because a previous recording was still converting. I realised that I had forgotten to select a location for the recording of my previous meeting, so that dialogue box was still open.
The error message said I could go to meetings > recordings to manually convert the recording. But I can't find that. On the app, I can go to meetings > recordings, but it only shows a few old meetings.