"Protected AAC audio file" playing fine, but "AAC audio file" not playing (on same album)

I have an album which I downloaded onto iTunes. I never had any issues with it until a few months ago (and, as with all things iTunes, no rhyme or reason why).

Some of the songs play fine, but some refuse to play.

The ones that refuse to play display a dialogue box stating that the computer is not authorised to play "said song" and would I like to authorise it. I authorise it (email and password) and it states that the PC is already authorised and goes straight back to the list of songs.

On closer inspection, the file type (kind) of the songs that will not play are "AAC audio file" and are .m4a files, whereas all the rest (that will play) are "Protected AAC audio file" and are m4p files.

in addition, the songs that play ok show as being purchased by me (but on an older account) but the ones that do not play do not display this information. I'm afraid I have no idea why.

I have attached screenshots of both for comparison.

I have tried to delete from iTunes, re-upload, changed the extension, all to no avail.

Short of rebuying the album on ebay ( i cant seem to find it on iTunes), can anyone suggest a solution.

Many thanks,

Richard