Local files won't show up on my iPhone

[ltalley0]

Hello all! I've had premium for almost two years now, and there is an artist that is on Spotify but a lot of his older music isn't on there, so I've been downloading them as local files onto Spotify from my computer. The files are stored on my computer, but also on my phone in iTunes as well. About a year ago I made a playlist on my computer where many of the songs were local files, and the songs showed up on both my phone and computer easily and played as well no problem. About a week ago I uploaded some more local files onto my computer and put them into a playlist and they show up on my computer fine but they don't even appear on my phone. 

I have been searching all week trying to figure out how to fix this problem to no avail, and I'm really confused because I had done this just a year earlier with no problems whatsoever. I have tried going through different forums on here, but many are outdated/don't solve my problem. I have turned on the download feature for this specific playlist on both my computer and phone and it doesn't solve the problem. Both my computer and phone are connected to the same wifi. I have tried plugging my phone into my computer but that doesn't work either.

I have even tried pressing play from my computer on one of the local file songs while spotify is playing on my iPhone, but a message pops up on my iPhone and says "Song not downloaded - if you own this song, you can sync it from your computer" and when I press on "learn more" it says to mark the playlist containing this song for Offline Sync on your computer or on this iPhone. I have no idea how to turn on Offline Sync, and I'm assuming it's different from downloading the playlist for offline streaming because that is already turned on for this playlist. I'm not sure what is up, but the instructions are very vague and I'm hoping someone can help!! Thanks a ton everyone!!