well, The only thing I can think to ask is if you purchased the DLC on the same xbox you're currently playing now? If you purchased and downloaded it on a different xbox, then the license for that DLC is attached to that xbox. As long as you are connected
to Xbox live, you can play any DLC you have a license for on any xbox. That's why it will allow you to play on this xbox. If you are playing offline, You can only use the DLC on the xbox it was downloaded to.
If this is not the case then I am just as stumped as you are.