Music re-authorisation loop

I purchased a selection of songs from the iTunes Store on my iPhone 7, and it was playing fine from my device. The next time I synced my iPhone with my iTunes library on my computer, it transferred the music. Since then I've re-synced and the music is no longer on my phone, though it's still in my Library on my computer.

When I try to play the songs on my computer, an error message appears that says 'Cannot Open: This content is not authorised'. When I attempt to play any of the songs on my computer, a notification appears that says 'This computer is not authorised to play [track title]. Would you like to authorise it?'. I've since entered my Apple ID and password when this prompt appears, and each time a message informs me that the computer was previously authorised, and has been successfully reauthorised. In spite of this, I am still unable to play these tracks, as the same error message displays no matter how many times I enter my Apple ID and password.

I only have one Apple ID, but the songs I purchased no longer appear in my iTunes Store account purchases so I can't delete them and re-download them. I have de-authorised my computers and re-authorised them.