My computer is authorized but some tunes won't synch

I've had a few ipods and a few computers that have crashed and been replaced, so it's not surprising things get a little messed up with Itunes and my current ipod nano. Right now, I have 11 out of about 1000 songs that won't synch to my nano. These 11 songs are among the many I have purchased from the Itunes store, so I do own them. I get a message saying they are not authorized, that I need to authorize this pc to have that happen. The pc is already authorized. I "reauthorize" it but it still won't work. I suspect this goes back to having an Itunes account that we no longer use. If that's true, how do I get Apple to move my songs to my authorized account, or what else would you suggest? Thanks for your time. /Janet