I have this issue too. Up until late July I had an iPhone X and at first it didn’t sync, but after awhile it just started working, and then it would sync within a few hours, usually when I wasn’t even using the phone. Sometimes it even downloaded them on my phone overnight while I was asleep haha. But in late June this year that phone broke, and I now have an iPhone 8 instead. Problem is the 8 won’t sync. It shows the files in the playlist because I have show unavailable songs enabled, but they’re greyed out and can’t be played. They work fine on my PC, and if I play one on there and then try to send it to my phone with the devices menu it just says “This song can not be played” or something and just puts the next song on that isn’t a local file. It never syncs either, I’ve had a playlist with a handful of them downloaded on my phone for almost a month now, and the local songs show a grey download arrow next to it like they want to download, but then never do. I’ve tried everything except hot spotting my phone to my PC because my area doesn’t have cell reception, so it wouldn’t do anything at all then. But I’ve tried everything I can think of and it doesn’t sync. Never had this problem until I got a new iPhone, and even the computer is still the same.
