Songs won't transfer to iPhone

So recently (around the time I joined my family's Apple Music account, actually), songs just stopped transferring to iTunes. I do what I usually do, select the song, select "add to playlist" and then select my iPhone, but absolutely nothing happens. I even resorted to syncing out of desperation, but that didn't do anything either, it didn't add any of my newly downloaded songs to my iPhone, and it didn't even delete the ones that aren't on my current computer.