Hi tzinaki,
Welcome to the Support Communities!
I would recommend deleting and redownloading the song that you are having an issue with on your iPhone:
You can delete individual songs, artists, albums, playlists, compilations and composers from your iPhone.
This information is on page 61 of the iPhone User Guide
manuals.info.apple.com/en_US/iphone_user_guide.pdf
Delete a song from iPhone: In Songs, swipe the song, then tap Delete.
The song is deleted from iPhone, but not from your iTunes library on your Mac or PC, or from iCloud.
Downloading past purchases from the App Store, iBookstore, and iTunes Store
http://support.apple.com/kb/ht2519.
Cheers!
Judy