You can’t stream every track that’s available in the iTunes Store. As I pointed out recently, there are many tracks that aren’t streamable. If an entire album isn’t available to stream, you won’t see it in Apple Music, but if only certain tracks are unavailable, then you’ll see the others when you select an album.
But it seems that Apple Music playlist curators aren’t aware of this. Here’s a playlist that came up in my For You recommendations:
As you can see, the first track is dimmed, meaning that you can’t stream it. If you click the Play button, the playlist starts from the second track; if you double-click the first track, nothing happens.