Some purchases refusing to transfer to ipod

I did try to look through previous questions but none of them either matched, or had suggestions that worked. Im sure this is a freq asked question, so forgive me for asking it ..... again 🙂

I had to completely reformat my ipod to upgrade to 5.0. Even though I backed up my ipod first, it erased a bunch of my purchased songs that I'd gotten several years ago. Ok, fine, at this point I just buy my MP3s somewhere else. The problem is, 100 songs wont sync, because they say my ipod isnt authorized for those purchases. I double checked (triple, quadruple, for that matter) and not only is my ipod authorized, it's the only ipod I have, so Im completely lost on how it wouldnt be authorized. The only thing I can think of is that I have more than 7 copies of the songs in question out there, and have thus hit my limit? The problem with this is ......1) I thought you could have your purchased music on as many ipods as you liked, with no limits (limits only being applied to seperate computers and burned CDs, etc). ........2) Ive checked several times to confirm, and only my desktop (the computer the library is on) is authorized and Im positive I didnt burn all of the songs to 6 additional CDs (one of the songs that wont transfer, for instance, is a song I hate and I am well and truly positive it has never been put on a CD, another computer, or anything. Im fairly certain I've never even listened to it a 2nd time).

Any help would be appreciated. Like I said, at this point Ive just taken to getting my MP3s somewhere else. Oh, does anyone know if there's a limit on how long you can re-download purchased songs? It currently shows I have no purchase history, and I know I did buy many songs which have now vanished from my library, but they were all bought at least 3 years ago (if not more).

Thanks a bunch.