What worked for me was logging into the Spotify desktop app and selecting "Show unavailable songs in playlists", which I believe is deselected by default.
There were quite a few local files that I had deleted from iTunes (or from iTunes playlists that I had deleted) that were still technically in my Spotify playlist. I think those are what Spotify on my iPhone was forever "Waiting to Download".
Syncing between Spotify Web, Spotify Desktop, and Spotify smartphone is a bit of a mess, imo.
Spotify > Preferences > Display Options
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