Halo Reach Achivements Problem

for some reason, my achievments do not register when i complete them…for example:

  1. I earned a Double Kill with a Shotgun yesterday, three times in one game

Both Barrels
Earn a Double Kill with the shotgun in multiplayer Matchmaking.

  1. Most of my commendations are in their silver state

Lemme Upgrade Ya
Advance any Commendation to its Silver state.

I completed many more matchmaking achievments, yet they do not register!!! any help???