Not authorsied to play certain songs

Hello

I have recently purchased an album of 16 songs from the Apple iTunes store and synched it to my iPod. I can play 12 of the 16 tracks with no problem. However and random selection of 4 tracks won't play as apparently my computer is not authorised to play them. When I try to authorise it by inserting my password and user name a message simply tells me that my computer is authorised and is the only one of my five authorised. I then try to play the track again and the same problem occurs. Any ideas please?