I have local files downloaded on my PC's Spotify. I added them to my playlist and the majority downloaded to my Android phone for offline play. That said, there are 3 songs that did not download to my phone. They are grayed out and do not have the download arrow next to them.
We'd advise you to first make sure everything with your cache is okay. You can do this by running a clean reinstall. It's very important to remove the devices for offline listening - just follow the each step in the guide. After that, you will have to download all of your music for offline use again.
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