Songs in iTunes library won't sync to playlists on iPhone6

I have an iPhone 6, running newly installed version 10.0.2. The last few times I have purchased music from iTunes (mostly prior to this update), I have had issues moving those items to my playlists. I can see and can play the song from my library, and I get a confirmation when I add to the playlist, but when I look at the playlist on the phone, the song isn't there. The items ARE on the phone, and playable, I just can't get the into the playlists.

I have synced the phone to my MacBook Pro (iOS 10.11.6) several times; the items in question are visible and playable from the playlist on the MacBook, they just never seem to make it to the playlists on the phone. I am nowhere near any capacity limits on the phone, and the songs even sync to the proper playlist on my ages-old 16gb iPod Nano. I don't use Apple Music. Very, very frustrated...can someone please offer a solution? Thanks!