Can't copy music from iTunes to iPhone - appears greyed out

I have a large iTunes library on my PC, around 1,000 albums, mostly ripped from CDs, some iTunes store purchases.

I manually copy music from iTunes to my iPhone (2020 SE), occasionally I have an issue where the music doesn't copy, although the tracks do appear on my iPhone but are greyed out. It's like it has copied the names of the tracks but not the actual files.

This can happen with music from CDs or digital purchases. I have to delete the tracks from the iPhone and try again. Usually a combination of restarting iTunes and restarting the iPhone sorts it out.

This time I have the issue and just can't get around it. It was only one week ago that I successfully downloaded an album from iTunes and copied to my iPhone, then with this latest purchase it just won't copy.

So frustrating. Help please.