This has been the case for years. You'll find many large artists that don't show up and many small artists that do and there often appears to be no rhyme or reason to it. And who shows up and who doesn't changes. There's really nothing you can do about it. Your best bet is to see if they come up in the audience insights tool and target similar artists with a strong likelihood of sharing fans.

If you were hell bent on getting in front of that audience then one thing you could try would be to target an artist similar to the artist in question but reference your ideal target artist in the copy. IE, "Following in the footsteps of artists like Lisa Hannigan, Karen Grace...".

By dangling that line you'd be insuring that you were targeting fans of that artist and then the algorithm could take over and find more of those fans for you. That said, that sounds like a bit of unnecessary work.