Weird warning message when I sync ipod

When I plug my ipod into my macbook pro I get this message...

"some of the items in the itunes library, including (etc etc), were not copied to the ipod because you are not authorized for them on this computer."
"To authorize this computer for items purchased from the iTunes store choose Store>authorise this computer."

When I do this, it tells me my computer is already authorized.
Can anyone help explain why this happens?
I've always found it very confusing plugging my ipod into more than one computer and wonder whether this has anything to do with it. I used to plug it in to my partner's computer before I got the new one and that caused me a few problems but I don't understand why I get this message.

Any help is much appreciated.

benjamin163 said:

I've always found it very confusing plugging my ipod into more than one computer and wonder whether this has anything to do with it.

Click to expand...

Actually, it has everything to do with it. You can only sync your iDevice (iPod, iPhone, iPad) to one library (at a time) which necessarily means syncing only on one computer.