iTunes files loaded on to new iMac keeps asking me to authorise the computer (and won't play) but when I do this it says that computer is already authorised. Help please!

I recently purchased a new I Mac (the old one's hard drive failed - fortunately I had backed up the old machine). I have successfully managed to load the old files including iTunes. However, when I now try to play iTunes TV shows I get a prompt to authorise the computer to play any of these shows. When I do the authorisation, I am told that the computer is already authorised ⚠ - and it still won't play. I have no parental controls in operation and have already tried deauthorising and then authorising all computers. Any ideas on how to break out of this cycle would be greatly appreciated!