Protected AAC

None of my Protected AAC tracks will now play. Get a messgae that the computer is not authorised although it is!

Anyway I authorise it and that gets accepted but when trying to play a track it says the computer is not authorised.

Have deauthorised and re-authorised with no effect.

Have checked with get info and the tracks were purcahsed with the same account (although the password may have changed).

Any ideas much appreciated.