I got my Gamerscore reset a couple of weeks ago due to me Modding it which I openly admit to but one thing is that I find it really difficult to regain my Achievements. I, before I modded it, had about 24K Gamerscore but I then modded it to around 45K. Obviously it was inevitable that it would get reset of which I am not bothered about but one thing that is really aggravating me is that I can get an achievement but it does not register with my account. MW2 as an example, I yesterday completed a couple of missions on Veteran, the achievement popped up. I checked the Gamerscore, still the same but the achievements had unlocked on the Game Achievement page. I find this really peculiar as it does actually give me the achievement but I never received the Gamerscore. Fine, I made a mistake but don't make it impossible for me to gain achievements.
Any help is appreciated in this area as I would like to get back onto achieving my 20K+ Gamerscore back.