Through a series of events stemming from today's iOS 8 update which I won't go into, I was shocked to find that some of my playlists had not been complete in all the time I've had Spotify on my iPhone. Namely, the songs in a playlist that were from my personal library.
And before I go on, I do understand that when a song is greyed out on the my phone's playlist, I need to sync the playlist to my desktop and it they will all be playable. I realize Spotify cannot always match the data of a song file from a personal library to their service. It is to be expected.
Here is what I do not understand. In some cases, the songs appear greyed out on the phone (as just described) but in other cases, the song simply does not show up on the phone at all. And that's what got me! I didn't know songs weren't syncing because they weren't greyed out... they just weren't there! What caught my eye was the fact an album looked really short. And as I dug, I kept seeing that in several playlists.
On the desktop, there is no consistency for why this occurs. I thought at first, if the song had a greyed out artist AND album name, those were the ones not showing up at all on my phone. If only artist OR album were greyed out, then it would appear on my phone as a greyed out song. But honestly, that was not 100%.
I ended up combing and comparing every playlist between desktop and phone. My solution was to create a playlist called "My Unsynced Music" and dumped all the grey and absent song files into it and synced that to my phone. (This method takes vastly less memory than syncing dozens of full playlists.) Then by virtue of them being on my phone from that playlist, they all lit up or appeared on the proper playlists I created.
I guess there's really no question here other than a plea for clarity and if there's a better way to work through this than what I came up with.