Music will not transfer to iPhone from MacOS Music app.

Hi all,

Something that has been frustrating me for a while now:

Sometimes when I import an album I've purchased into the Music app on my Mac Mini, and then drag that album over to my iPhone (because it doesn't automatically add it to my iPhone), nothing happens.

For example, today I purchased two albums from Bandcamp (same band for both albums), and after I imported them into the Music app, I grabbed each album, and dragged it to my iPhone, which appears under Devices in the side menu. One of those albums transferred to my phone, the other did not. No matter how many times I drag it over or reboot the app/phone, it will not transfer.

Why in the world does this happen? What is preventing certain albums from being transferred, and how do you fix this annoying problem?

Thanks in advance.