Cannot download full music library onto iPhone 6SPlus

Purchased new iPhone 6 Plus 16GB to upgrade from my 8GB iPhone 5. Usually I could drag my music, purchased or downloaded, into the iTunes Library, and could sync my iPhone 5 with all the music it had space for.

The same process worked for my old iPhone 6S from 2016 as well, but is not working for this iPhone 6S Plus, which says “iCloud Music Library is On” but only offers to sync voice memos, and is only syncing 132 of my 1,000+ songs.