So my woe with this.
I don't often need to call Apple, but probably a handful of times in ten years, but this has really irritated me. Genius used to work great. Now it doesn't. In my case it works as it always did for anything more than two years old, whether it came from CD or has been an iTunes purchase. After that, it is hit and miss, and anything from the past year it doesn't do. I made a call to Apple support a fortnight ago, and after we had turned the Genius on and off, de-authorised and re-authorised, they suggested reinstalling iTunes, and Genius then worked. It generated a playlist from a song from the AM by the Arctic Monkeys. I didn't try any others.
Now I came to generate another playlist today, and it doesn't work again. Except, get this, it does work for the specific song, and only that song from the AM album.
I then spent 2 hours on the phone to Apple Techs was escalated, twice, and diagnostics have now gone off. We tried it on different Macs, different users, same results. Tech said it must be either a bug in iTunes, or a server-side bug at Apple.
I get a report back in four days - so I will post if it is of any use. Really frustrating. If anyone has the solution to this please post it!