- last post: 01.01.0001 12:00 AM PDT
Sometimes, you'll get disconnected from LIVE, and when you do, it doesn't count the gameplay that is done until you reconnect again. If you do reconnect, you gotta go through the part of the campaign that you did while you were disconnected, in order for it to be recorded as "done".
This problem actually happened to me on Gears of War (PC). So I went through the entire game again, and at the end of the third level, I unlocked the achievement saying I beat the game on "Legendary" (called Insane in GoW). So that probably means that somewhere between the beginning of the game and the end of the third level, I disconnected and it didn't record some gameplay as being "done".
Try and make sure you have a solid connection. Check Microsoft's Knowledge Database on improving connectivity, and make sure you open the specified ports in your firewall/modem/router. Also, don't run additional applications that require a connection, with the exception of important things like antiviruses and antispyware.
If you connect wirelessly to a router, make sure you have "excellent" as your signal strength at all times. If you don't, try moving the location of the router or your gaming location, and make sure you don't play near appliances such as phones that operate on the same frequency as your router.
Maintenance your computer often; scan for viruses and spyware, clean out your internet cache, delete temporary files, and repair your registry.
That's all I can think of right now, so do a thorough lookover of the article that Microsoft provides in their Knowledge Database.
P.S. If you complete the game on Hard, it will unlock both the 'Completed on Hard' and 'Completed on Normal' achievements at the same time. And completing the game on Legendary will unlock Legendary, Hard, and Normal at the same time, though I'd recommend finishing it on Hard first.
[Edited on 10.30.2008 11:50 PM PDT]