So I went looking for a song on my Nexus Player in Google Play Music, and it wasn't there. I spot checked a few other albums in my library (both songs I uploaded and songs I added from All Access), and a lot of albums were completely missing or missing a number of tracks. My library looks and plays just fine on the website and from my phone and tablet. Clearing data and cache from Play Music doesn't fix the problem.
I am running Android 5.1.1 (rooted) on my Nexus Player, and Google Play Music v5.9.1854R.1904527.
Maybe a longshot, but could it be a location-based issue? As in, are there any restrictions on the material in a certain country, and somehow, your NP thinks you are in that country?