Halo 3 Completed but No Gamerscore Received :(

Hi, basically as the topic name says ive completed Halo 3 on heroic (watched the end cut-scene then the credits then the final cut-scene then its taken me back to the menu). After all this ive received no gamerscore for it :(

.im signed into an xbox live profile

.repeated the end mission 3 times still nothing

.definatley not got the acheivement for it im 100% positive of that

So am i missing something please help and it may sound sad that im crying over 125 gamerscore but i want it XD

Did you complete all missions on the same difficulty? If not you won't recieve the achievement.

Both times I played through Gears and Gears 2 I never got the co op achievement but my brother did. Pretty awesome.immortality20