Why are some songs greyed out whilst others are in black?

That’s a perennial issue, one that I get from time to time, and one that I still fail to get to the bottom of. They’re usually downloaded tracks, but I’m not sure why it happens. There’s various threads about it on the Apple support forum.
There are some suggestions here:
https://www.tuneskit.com/apple-music-tips/fix-itunes-songs-greyed-out.html
Caveat: this is from a software vendor selling a product, but they do have suggestions that don’t need their product.