Honestly this issue is more for Microsoft. The fact is that when you purchase a game through Xbox live it is tied to the gamertag which purchased it, and to the Xbox it was originally purchased on. Have you gotten a new Xbox recently? If so then you'll need to "transfer licenses" to the new Xbox. If you haven't then this sounds like some kind of a glitch in the system.
I would suggest looking here:http://support.xbox.com/en-us/xbox-live ... ve-content
Hopefully this can help you out.