I tried to download these from the Xbox Live marketplace a little bit ago and it said that it couldn't download it, without giving any reason why. Trying to download these spent my MS points too.

I had this problem when they first came out, I no longer have a 360 and been years since I had the problem sorted but I had to contact the customer care service. i know it involved setting up a new profile or something.

Similar thing has happened to me, I got a new Xbox and did the whole gamertag recovery thing to get my stuff back, and then borrowed one of those memory transfer thingies, and everything got transferred except the DLC. I re-installed SI and KOTN because I have the GOTY edition, but the other DLC still don't work. Any chance you have started a new profile or recovered a profile or anything like that?