Hey jasmclar,
So to confirm, you're only having this issue with one specific song. Is that correct? If yes, we do see that you tried deleting the song and then redownloading it.
Is this a purchased song, or do you have an Apple Music subscription, and are redownloading from the Apple Music Catalog?
Verify the steps are followed from the resource which applies:
•Purchased song: Redownload music - Apple Support
•Apple Music subscription, can you play the song from the Apple Music Catalog? If so, redownload from catalog: Add and download music from Apple Music - Apple Support
For both options, you'll want to make sure you are connected to Wi-Fi or cellular data. If cellular data, verify is enabled for the Music app at Settings > Cellular.
Let us know how it goes!
Best regards.