Why do I have to keep re-downloading halo 3 map packs?

[geo2329]

For some reason every time I log on to Xbox live the game says I don't have the maps and I have to download them again everytime I want to play
Downloaded it, and they are on my hard drive, but it says i don't have them

 

[deathbot]

do you do it by cd or download
by cd save to hard drive
by download you still save to hard drive